Adds pages for GitLab Shell to development dir

Add navigation item to global navigation

• Link to the merge request that introduced the new page: multiple
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

MRs that introduced the new pages

• gitlab!107755 (merged) introduced doc/development/gitlab_shell/index.md
• gitlab!107757 (merged) introduced doc/development/gitlab_shell/process.md
• gitlab!107760 (merged) introduced
  • doc/development/gitlab_shell/features.md
  • doc/development/gitlab_shell/gitlab_sshd.md

Related issues

Related to Move Shell documentation into the main develope... (gitlab-shell#596 - closed) where we're tracking all the work.