[docs]: Update contribution docs

[holmanb]

Documentation request

The code contribution page could use some attention.

1. Feature flags deserves its own page - that is not a code contribution-specific thing to document.
2. Conventional commits should be documented on this page, linked from the "first PR" page.
3. Other resources is not relevant to code contributions, find a new home.

[blackboxsw]

Agreed on these restructuring points. I do think add some content on conventional commits docs in the first PR page would be a good refefence as the link in the wall of text in our pull request template is easily overlooked/ignored.