Docs: Move CONTRIBUTING.md to docs/

[ockham]

Changes proposed in this Pull Request

• Docs: Move CONTRIBUTING.md from .github/ to docs/

The motivation for this is #35833 -- to enable webpack loaders to find this file (in a non-hidden directory).

GitHub should continue to be able to find this file:

To help your project contributors do good work, you can add a file with contribution guidelines to your project repository's root, docs, or .github folder. When someone opens a pull request or creates an issue, they will see a link to that file.

(Source)

Testing instructions

• Go to calypso.localhost:3000/devdocs, and click on "Contributing" in the sidebar. Verify that the link still works.
• Grep the codebase to verify there are no other references to .github/CONTRIBUTING.md.

[matticbot]

Test live: https://calypso.live/?branch=move/contributing-md-to-docs

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~3 bytes removed 📉 [gzipped])

name     parsed_size           gzip_size
devdocs        -12 B  (-0.0%)       -3 B  (-0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[lancewillett]

Any objections or gotchas, @Automattic/team-calypso? Looks sane to me.

[tyxla]

Needs a rebase, but other than that LGTM 👍

Will take over to land it, as Bernie is AFK these days.

[tyxla]

This LGTM, thanks! Going to 🚢 it.