docs: remove remaining orphaned placeholder pages

Follow-up to the previous placeholder cleanup. These four pages are the
last of the "TODO" stubs carried over from the subtree merges that were
never referenced by the sidebar, any inbound link, or an autogenerated
directory:

- docs/07-standards/index.md
- docs/01-getting-started/containerization.md
- docs/01-getting-started/overview.md
- docs/01-getting-started/virtualization.md

Each contains only a heading and a "TODO" marker. The getting-started
directory keeps preinstall-checklist.md, which holds real content and is
wired into the sidebar in a separate change.

No live link or sidebar entry referenced any of these files, so the
build is unaffected.

DocImpact
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jan Klare <klare@osism.tech>

Author: jklare