[libc][docs] Parse inline macro_value from YAML in docgen (#189118)

The docgen script was previously hardcoded to assume all implemented
macros must be placed in a *-macros.h header. This updates docgen to
read inline macro_value properties directly from the source YAML files,
correctly recognizing them as implemented.

GitOrigin-RevId: d6ff5e77781dfe9623e9eff10907dc45fab8ba3f