Docs: Add docs on services (#19741)

* Add docs on services

* Apply suggestions from code review

Co-Authored-By: gotjosh <josue.abreu@gmail.com>

* Fix review comments

* Extract communication to its own document

* Minor fixes

* Minor fix

* Apply suggestions from code review

Co-Authored-By: gotjosh <josue.abreu@gmail.com>

* Add comment on exported field

contribute/architecture/communication.md

+# Communication
+
+Grafana uses a _bus_ to pass messages between different parts of the application. All communication over the bus happens synchronously.
+
+There are three types of messages: _events_, _commands_, and _queries_.
+
+## Events
+
+An event is something that happened in the past. Since an event has already happened, you can't change it. Instead, you can react to events by triggering additional application logic to be run, whenever they occur.
+
+> Because they happened in the past, event names are written in past tense, such as `UserCreated`, and `OrgUpdated`.
+
+### Subscribe to an event
+
+In order to react to an event, you first need to _subscribe_ to it. 
+
+To subscribe to an event, register an _event listener_ in the service's `Init` method:
+
+```go
+func (s *MyService) Init() error {
+    s.bus.AddEventListener(s.UserCreated)
+    return nil
+}
+
+func (s *MyService) UserCreated(event *events.UserCreated) error {
+    // ...
+}
+```
+
+**Tip:** Browse the available events in the `events` package.
+
+### Publish an event
+
+If you want to let other parts of the application react to changes in a service, you can publish your own events:
+
+```go
+event := &events.StickersSentEvent {
+    UserID: "taylor",
+    Count:   1,
+}
+if err := s.bus.Publish(event); err != nil {
+    return err
+}
+```
+
+## Commands
+
+A command is a request for an action to be taken. Unlike an event's fire-and-forget approach, a command can fail as it is handled. The handler will then return an error.
+
+> Because we request an operation to be performed, command are written in imperative mood, such as `CreateFolderCommand`, and `DeletePlaylistCommand`.
+
+### Dispatch a command
+
+To dispatch a command, pass the object to the `Dispatch` method:
+
+```go
+cmd := &models.SendStickersCommand {
+    UserID: "taylor",
+    Count: 1,
+}
+if err := s.bus.Dispatch(cmd); err != nil {
+  if err == bus.ErrHandlerNotFound {
+    return nil
+  }
+  return err
+}
+```
+
+**Note:** `Dispatch` will return an error if no handler is registered for that command.
+
+**Tip:** Browse the available commands in the `models` package.
+
+### Handle commands
+
+Let others parts of the application dispatch commands to a service, by registering a _command handler_:
+
+To handle a command, register a command handler in the `Init` function.
+
+```go
+func (s *MyService) Init() error {
+    s.bus.AddHandler(s.SendStickers)
+    return nil
+}
+
+func (s *MyService) SendStickers(cmd *models.SendStickersCommand) error {
+    // ...
+}
+```
+
+**Note:** The handler method may return an error if unable to complete the command.
+
+## Queries
+
+A command handler can optionally populate the command sent it. This pattern is commonly used to implement _queries_.
+
+### Making a query
+
+To make a query, dispatch the query instance just like you would a command. When the `Dispatch` method returns, the `Results` field contains the result of the query.
+
+```go
+query := &models.FindDashboardQuery{
+    ID: "foo",
+}
+if err := bus.Dispatch(query); err != nil {
+    return err
+}
+// The query now contains a result.
+for _, item := range query.Results {
+    // ...
+}
+```
+
+### Return query results
+
+To return results for a query, set any of the fields on the query argument before returning:
+
+```go
+func (s *MyService) FindDashboard(query *models.FindDashboardQuery) error {
+    // ...
+    query.Result = dashboard
+    return nil
+}
+```

contribute/architecture/services.md

+# Services
+
+A Grafana _service_ encapsulates and exposes application logic to the rest of the application, through a set of related operations. 
+
+Before a service can start communicating with the rest of Grafana, it needs to be registered in the _service registry_.
+
+The service registry keeps track of all available services during runtime. On start-up, Grafana uses the registry to build a dependency graph of services, a _service graph_.
+
+Even though the services in Grafana do different things, they share a number of patterns. To better understand how a service works, let's build one from scratch!
+
+## Create a service
+
+To start building a service:
+
+- Create a new Go package `mysvc` in the [pkg/services](/pkg/services) directory.
+- Create a `service.go` file inside your new directory.
+
+All services need to implement the [Service](https://godoc.org/github.com/grafana/grafana/pkg/registry#Service) interface:
+
+```go
+type MyService struct {
+}
+
+func (s *MyService) Init() error {
+    return nil
+}
+```
+
+The `Init` method is used to initialize and configure the service to make it ready to use. Services that return an error halt Grafana's startup process and cause the error to be logged as it exits.
+
+## Register a service
+
+Every service needs to be registered with the application for it to be included in the service graph.
+
+To register a service, call the `registry.RegisterService` function in a `init` function within your package.
+
+```go
+func init() {
+    registry.RegisterService(&MyService{})
+}
+```
+
+`init` functions are only run whenever a package is imported, so we also need to import the package in the application. In the `server.go` file under `pkg/cmd/grafana-server`, import the package we just created:
+
+```go
+import _ "github.com/grafana/grafana/pkg/services/mysvc"
+```
+
+## Dependencies
+
+Grafana uses the [inject](https://github.com/facebookgo/inject) package to inject dependencies during runtime. 
+
+For example, to access the [bus](communication.md), add it to the `MyService` struct:
+
+```go
+type MyService struct {
+    Bus bus.Bus `inject:""`
+}
+```
+
+You can also inject other services in the same way:
+
+```go
+type MyService struct {
+    Service other.Service `inject:""`
+}
+```
+
+**Note:** Any injected dependency needs to be an exported field. Any unexported fields result in a runtime error.