Adds changelog structure, contributor list, and PR template

[louislelay]

This PR adds and improves the changelog structure, and introduces a contributors list:

• Added a global CHANGELOG.md to track repo-wide updates
• Moved changelog sections from each model's README.md into dedicated CHANGELOG.md files, and linked them from the corresponding READMEs (created missing changelogs where needed)
• Created a CONTRIBUTORS.md listing all contributors alphabetically
• Added a PR template under .github/pull_request_template.md to remind contributors to update changelogs and the contributors list
• Updated CONTRIBUTING.md to include new expectations
• Also cleaned up some formatting in the model README files

A few notes:

• In the contributors list, I separated core contributors (Google DeepMind) from the community based on names I recognized so it will likely need to be adjusted in case I missed anyone!
• In the global changelog, I only added the Menagerie release date and this contribution. Wasn’t sure if previous changes were meant to be backfilled there.

Fixes: #81

[kevinzakka]

Amazing job @louislelay, I know doing all these changes must not have been easy!

[louislelay]

Thanks @kevinzakka, there were quite a few files to go through haha!