DOP-6233: remove OrphanedPage diagnostic

[KamilKhxn]

Ticket

DOP-6233

Notes

Removes the OrphanedPage diagnostic. The check walked only toctree directives
reachable from contents.txt/index.txt, so pages reachable solely through the
unified ToC were reported as orphans even though they were in the ToC. Orphan
detection is now handled by a separate script that reads the unified ToC.

No AST or schema change — the :orphan: page option is still parsed and
serialized as before; only the warning is gone. Also updates
test_silencing_diagnostics, which used OrphanedPage merely as its sample
diagnostic, to silence DocUtilsParseError instead.

README updates

• • This PR introduces changes that should be reflected in the README.md and/or HACKING.md, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README.md and/or HACKING.md

[KamilKhxn]

Question, is the changelog.md necessary?