[DOCUMENTATION] Add GitHub Conventions Document

[standard-librarian]

Add GitHub conventions document for documenting existing conventions and add new conventions for issues and PRs.

Tasks

• Document the existing convention for:
  • Branches
  • Commit title
  • Merge commit message
  • PR title
  • Issue title
• Add issue templates
• Add PR templates
• Add atomic commit rules and body

References

• https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/about-issue-and-pull-request-templates
• https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository