libstdc++: Improve C++14 and C++17 status docs

libstdc++: Improve C++14 and C++17 status docs

This adds a full table of contents for the C++14 and C++17 standards,
with status for each part.

For C++14 the list of proposals is removed, as it adds little value now
that everything is supported. For C++17 the table of proposals is
retained, because it documents he feature test macros for the features.

	* doc/Makefile.am (xml_sources_manual): Add missing XML files.
	* doc/Makefile.in: Regenerate.
	* doc/xml/manual/status_cxx1998.xml: Refer to "this section" instead
	of "this page".
	* doc/xml/manual/status_cxx2011.xml: Formatting and other corrections
	to the C++11 status table.
	* doc/xml/manual/status_cxx2014.xml: Replace list of C++14 feature
	proposals with table matching contents of the C++14 standard.
	* doc/xml/manual/status_cxx2017.xml: Add table matching contents of
	the C++17 standard.
	* doc/html/*: Regenerate.