Add contributing guidelines and organize developer documentation

[MisRob]

Summary

• Adds CONTRIBUTING.md (including its copy in README.md)
  • The structure, placement, and content is copied from the corresponding Kolibri and KDS files (after their latest update Development and contributing guidelines updates kolibri#11560)
• Removes development related information from README.md in favor of adding index file and organizing all content into chapters of the development documentation directory
  • To make contributing guidelines visible
  • To make locating information easier
  • To align documentation structure with Kolibri and KDS
• Also updates config.yml so that redirects to Discussions and LE forum are available when opening a new issue
  (screenshot from Kolibri)

Reviewer guidance

• Preview CONTRIBUTING.md and README.md
• Preview the new index file and organization of the developers docs
  • Note that the content of developer documentation wasn't changed (except of a few minor tweaks like some section titles and headings levels). Please focus on the new structure in the scope of this PR only (I wouldn't have bandwith to follow-up on cleaning up content anyways)

[bjester]

LGTM

[vkWeb]

I think we should annotate that the "With docker" setup is recommended from our side. From what I have observed, open source developers tend to use recommended methods, so that less issues pop up later down the road. Thoughts @bjester @MisRob ?

[bjester]

@vkWeb Yeah that is the recommended setup, so it's worthwhile to mention that

[MisRob]

Thanks both, I mentioned that Docker setup is recommended in a few places. It's also the page where you will be taken from the contributing guidelines.