The RST-to-Markdown migration (#206181) converted the RST flag
`:numbered:` into `:numbered: true`.
MyST parses the toctree `numbered` option as `int_or_nothing`, so the
string `true` fails with:
```
'toctree': Invalid option value for 'numbered': true:
invalid literal for int() with base 10: 'true'
```
This breaks the `-W` (warnings-as-errors) `docs-clang-html` build.
Make `numbered` a valueless flag, which MyST accepts (equivalent to the
original RST behavior of numbering all levels).
Assisted-By: claude
Balázs Benics
1d577d28b2