[patch] Update libstdc++ FAQ and ensure stable anchors in HTML docs

[Jonathan Wakely]

This fairly tedious patch refreshes the FAQ, including adding some
notes saying "This answer is old and probably no longer relevant" to
several answers referring to problems in the GCC 3.x era.

The rest of the patch adds xml:id attributes to several DocBook
elements to ensure they have stable anchors in the generated HTML.
I've also updated doc/Makefile to tell DocBook to generate stable IDs,
so we get anchors like <a id="id-1.3.6.3.6.10.9"> instead of one like
<a id="idm269884311200"> which changes every time you regenerate the
docs. This will significantly reduce the churn in commits which
regenerate the HTML.

Finally, I've unset the DOT_FONTNAME Doxygen setting, to stop Doxygen
warning that it no longer ships with that font.

Tested x86_64-linux, with DocBook 1.78.1 (the generate.consistent.ids
parameter needs 1.77.1 which is two years old now).
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