Document how to use the Group Extension API

[imath]

@boonegorges wrote a great explanation on the codex about it a while ago, I believe most of the work is copying in Markdown this content inside /docs/developer:

​https://codex.buddypress.org/developer/group-extension-api/

Steps:

• Create a new entry in https://github.com/buddypress/buddypress/blob/master/docs/developer/manifest.json
• Create a new directory eg: /docs/developer/group-extension
• Put the content in /docs/developer/group-extension/README.md.

Adds a documentation resource to explain how an Add-on can use the BP Group Extension API to add a group extension to BP Groups. It also informs about the 12.0.0 changes and how to preserve backward compatibility for previous versions of BuddyPress.

NB: this PR was exclusively built using the GitHub website 😉

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8941

[imath]

In 13527:

Document how to use the Group Extension API

• Update the ​Codex resource with new configuration requirement needed by the BP Rewrites API
• Update code examples
• Add screenshots to code examples

Fixes #8941
Closes ​https://github.com/buddypress/buddypress/pull/137