GitLab

* Updates the default branch from master to main

* Links from `master` to `main`

* Typo

* Update examples/flax/README.md
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

- mostly stylistic streamlining
- removed 'additional context' sections. They seem to be rarely used and might cause confusion. If more details are needed, users can add them to the 'details' section

- add 'your contribution' section
- add code formatting link to 'additional context'