Docs: E2E (#27461)

* Separated generalized E2E docs from that of core

… and rewrote some minor things to be more concise for faster reads.

* Added plugins E2E document

contribute/style-guides/e2e-core.md

+# End-to-End Tests for core Grafana
+
+This document is specific to the [Grafana repository](https://github.com/grafana/grafana). Be sure that you've read the [generalized E2E document](e2e.md).
+
+## Commands
+
+- `yarn e2e` Creates an isolated `grafana-server` home under _\<repo-root>/e2e/tmp_ with provisioned data sources and dashboards. This copies locally build binary and frontend assets from your repo root so you need to have a built backend and frontend for this to run locally. The server starts on port 3001 so it does not conflict with your normal dev server.
+- `yarn e2e:debug` Same as above but runs the tests in chrome and does not shutdown after completion.
+- `yarn e2e:dev` Same as above but does not run any tests on startup. It lets you pick a test first.
+
+If you already have a Grafana instance running, you can provide a specific URL by setting the `BASE_URL` environment variable:
+
+```shell
+BASE_URL=http://172.0.10.2:3333 yarn e2e
+```
+
+The above commands use some utils scripts under [_\<repo-root>/e2e_](../../e2e) that can also be used for more control.
+
+- `./e2e/start-server` This creates a fresh new grafana server working dir, setup's config and starts the server. It will also kill any previously started server that is still running using pid file at _\<repo-root>/e2e/tmp/pid_.
+- `./e2e/wait-for-grafana` waits for `$HOST` and `$PORT` to be available. Per default localhost and 3001.
+- `./e2e/run-suite <debug|dev|noarg>` Starts cypress in different modes.
+
+## Test suites
+
+All the integration tests are located at _\<repo-root>/e2e/suite\<x>/specs_. The page objects and reusable flows are in the [_\<repo-root>/packages/grafana-e2e_](../../packages/grafana-e2e) package.  

contribute/style-guides/e2e-plugins.md

+# End-to-End Tests for plugins
+
+Be sure that you've read the [generalized E2E document](e2e.md).
+
+## Commands
+
+- `yarn test:e2e` will run [Grafana's E2E utility](../../packages/grafana-e2e) against an already running Grafana server.
+- `yarn test:e2e:update` will run `test:e2e` but instead of asserting that screenshots match their expected fixtures, they'll be replaced with new ones.
+
+Your running Grafana instance can be targeted by setting the `CYPRESS_BASE_URL`, `CYPRESS_USERNAME` and `CYPRESS_PASSWORD` environment variableS:
+
+```shell
+CYPRESS_BASE_URL=https://localhost:3000 CYPRESS_USERNAME=admin CYPRESS_PASSWORD=admin yarn test:e2e
+```
+
+## Test suites
+
+All tests are located at _\<repo-root>/cypress/integration_ by default.
+
+## Things to test
+
+- Add data source (if applicable)
+- Add panel
+- Edit panel
+- Annotations (if applicable)
+- Aliases (if applicable)
+- Template variables
+- "Explore" view