* docs/reference/pangomm_{header,footer}.html_fragment: Delete
files.  These custom fragments were an unnecessary maintenance
hassle.  Also, the files were out of date with respect to the
encoding and CSS classes used by Doxygen.
* docs/reference/Doxyfile.in (HTML_HEADER), (HTML_FOOTER): Set
to the empty string.
(PROJECT_NUMBER): Substitute @PACKAGE_VERSION@.  It does look
nicer in the generated HTML pages, and is probably also less
confusing.
(SORT_GROUP_NAMES): Enable for predictability.
* docs/Makefile.am (dist_noinst_DATA): Remove deleted HTML
fragment files.