m.htmlsanity: properly use DEFAULT_LANG and :lang: metadata.

m.htmlsanity: properly use DEFAULT_LANG and :lang: metadata.

There's a patch to Pelican at https://github.com/getpelican/pelican/pull/2256
which propagates DEFAULT_LANG to docutils, but until that's accepted
and released, I'm detecting its presence and falling back to
monkey-patched version that enables the same from within the
m.htmlsanity plugin.

Besides that, the per-page/per-article :lang: metadata are also taken
into account. The docs were updated to reflect this.