docs: update information about GitLab and GitHub

Samuel Allanrequested to merge
samuel/forge-notes into master

Description

  • clarify that we still prefer GitLab for new projects wherever possible
  • clarify reasons to use GitHub
  • update instructions for adding teams to GitHub repos
  • general rewording for readability

Supporting information

See discussion on https://forum.opencraft.com/t/migrate-to-github/2149

Private-ref: https://tasks.opencraft.com/browse/BB-11015

Testing instructions

  • verify this doesn't change any policy, but just restates it for better clarity
  • verify the changes render as expected
  • verify everything is readable and accurate

Merge request reports