Update CONTRIBUTING.md (go-gitea#24492)

Previously, the `CONTRIBUTING` was severely outdated in certain aspects
such as workflows.
These sections have been brought up to date.
Furthermore, the `CONTRIBUTING` now mentions the TOC, how it is
structured, elected, and its duties.

---------

Signed-off-by: jolheiser <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>
Co-authored-by: Jason Song <[email protected]>
Co-authored-by: techknowlogick <[email protected]>
Co-authored-by: delvh <[email protected]>