Restore built HTML docs to the sdist

The pyproject migration in 1e439f6 rewrote the doc line in MANIFEST.in
and dropped the *.html pattern along with the examples-era file types it
was cleaning up.  The Sphinx documentation build emits its pages as
*.html directly into doc/ (via the dist-html target, which runs
sphinx-build into ..), so removing that pattern caused python setup.py
sdist to ship the doc/ tree with its stylesheets and scripts but none of
the actual HTML pages.  Restore *.html to the recursive-include so the
full documentation build is packaged again.

Change-Id: Ia6eb00edfaf000462360d08cd9bc59c3af5c9d4d

Author: zzzeek

MANIFEST.in

@@ -1,4 +1,4 @@
-recursive-include doc *.css *.txt *.js *.png *.py Makefile *.rst
+recursive-include doc *.html *.css *.txt *.js *.png *.py Makefile *.rst
 recursive-include examples *.py *.html
 recursive-include test *.py *.html *.mako *.cfg