documentation/doxygen: robustly skip unexpected XML files.

documentation/doxygen: robustly skip unexpected XML files.

This skips the Doxyfile.xml from 1.8.19 (without a warning, to avoid
polluting the output for everyone running newer Doxygen), and
additionally also all other files that don't have the expected root or
immediately following tags (with a warning, because that's likely
something to act on).

These checks used to be inconsistent between extract_metadata() and
parse_xml() (resulting extract_metadata() to continuing further and
dying on some other exception, sigh) and also was an assert instead of a
graceful handling.

A new test case is added to verify that all expected messages are
printed, all unexpected files are skipped but also the expected files
are not skipped. The warnings/errors are also printed just once, not
once during metadata extraction and once during actual page generation.

Co-authored-by: crf8472 <crf8472@web.de>
Co-authored-by: SRGDamia1 <sdamiano@stroudcenter.org>