Create new subsection for signing commits

Add navigation item to global navigation

• Link to the merge request that introduced the new page:
  • Create new central home for signed commits (gitlab!130465 - merged) added doc/user/project/repository/signed_commits/index.md
  • Move signing methods pages to new directory, re... (gitlab!130775 - merged) adds three child pages:
    • doc/user/project/repository/signed_commits/gpg.md
    • doc/user/project/repository/signed_commits/ssh.md
    • doc/user/project/repository/signed_commits/x509.md
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related

• Related to Create new central home for signed commits (gitlab!130465 - merged) which created the new directory
• Related to 384471 Refactor commit signature docs into a si... (gitlab!111348 - closed) where this all started long ago
• Related to Move signing methods pages to new directory, re... (gitlab!130775 - merged) which moves the last three pages
• Related to Refactor commit signature docs into a single page (gitlab#384471)