* configure.ac: AC_SUBST() GLOM_ABI_VERSION_UNDERLINED so we can use it
    in docs/pyglom_reference/Makefile.am. Mention these files that will be
    genereated form .in files with the ABI and version numbers substituted.
    Check for sphinx-build, though this should only matter when building docs.
	* docs/pyglom_reference/conf.py.in:
	* docs/pyglom_reference/index.rst.in: Use ABI and version variables instead
    of hard-coding. Add short introductory text with links.