Replace deprecated `recommonmark` with MyST-Parser

[radiantly]

I have made things!

Docs generation now uses MyST-Parser instead of the deprecated recommonmark to parse .md files.

Other changes:

• removed m2r package as it is no longer required
• remove monkey patches relating to m2r
• bump copyright year to 2021

Checklist

• [x] I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• [ ] I have created at least one test case for the changes I have made
• [ ] I have updated the documentation for the changes I have made
• [ ] I have added my changes to the CHANGELOG.md

Related issues

Closes #1082

[radiantly]

Done, but I think there's a bit of an issue when running doc8 - however, sphinx builds perfectly, so I'm not sure what exactly the problem is.

[thepabloaguilar]

Closing this PR since #1082 have been resolved!