[patch] create xrefs from .tely

[Graham Percival]

The attached patch lets me build xrefs by looking at
  Documentation/*.tely
directly, instead of looking at the corresponding
  Documentation/out-www/*.texi
files.

This would be very nice for the website, since we can't build all of
lilypond on the server.  (the alternative is to generate the xrefs
locally, upload the .xref-map files, and hope that they don't change
very often... this isn't quite as terrible as it sounds at first,
since we need to do this for the examples and pictures anyway, but it
would still be nice to avoid this)

I tested it out, and it seems to work, but I don't particularly know
how xrefs work, so I thought I should ask if it's ok.

Cheers,
- Graham

0001-Doc-build-allow-xref-map-creation-from-i-tely.patch
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