Deprecate gdk thin in favor of gdk rails s

What does this Merge Request do and why?

Deprecates gdk thin command in favor of gdk rails s.

Merge Request checklist

• [NO] This change is backward compatible.
  • If not, please include steps to communicate to our users.
  • Deprecation warning is included in the MR and our users can follow it.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [n/a] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Doc

pry and advanced installation are updated.

Docs on installation of relevant gems

Defers to #1511 (closed) (but already prepared in !2592 (merged))

Issues etc.

Closes #1512 (closed)

Blocked by !2576 (merged), gitlab!90958 (merged)

Updates db1a3d1e, !531 (merged), !800 (merged), and !1693 (merged)

Closes #183 (closed)