doc: exempt web from .xref-map dependency
As the 'web' document doesn't link directly to Learning Manual or Notation Manual, it is not necessary to build all xref maps as a prerequisite.
This reduces the overhead of the "make website" target.
As the 'web' document doesn't link directly to Learning Manual or Notation Manual, it is not necessary to build all xref maps as a prerequisite.
This reduces the overhead of the "make website" target.