* Replace bash/json with http in http_api codeblocks

Since http is used elsewhere on the API docs and makes the
markdown rendering look different on the blocks that use bash/json

* Fix the annotations API link on 'Features'

- Noticed the Features/Dashboard Features/Annotations link pointing to
the HTTP API was linking itself instead of '../http_api/annotations.md'.
- Also add a link on Dashboard Export and Import to its HTTP API.

* Add missing links to features in API Index

Noticed there was 3 missing references in the API docs index.
- https://grafana.com/docs/grafana/latest/http_api/playlist
- https://grafana.com/docs/grafana/latest/http_api/datasource_permissions
- https://grafana.com/docs/grafana/latest/http_api/external_group_sync

Last two are Grafana Enterprise -only, so created a separate subsection
for them.