List project contributors in the app

[lwouis]

After #245 we display credits for third-party libraries. We don't display any credits for contributors to the actual project.

I would like to add the list of contributors from github and from poeditor. I could be just a list of nicknames. It could be sorted by category, or list commits. It could link or not to the author github account or email. A lot is possible here in terms of UI.

[lwouis]

I have added a bash script which lists contributors in docs/CONTRIBUTORS.md.

I have only kept usernames, in order to preserve privacy (i.e. no emails). For github contributors, I have linked to their github profile as this is public information and promotes them.

I added a check in CI so that the markdown file is always up-to-date without needing to remember to run it. The check also ensures other files are up-to-date: localization reference file, localizations files in each language, and cocoapods libraries.

I'm keeping this ticket open because I would like to add that info in-app, probably merging it with the existing Acknowledgments preference tab.