docs: move checklinks to a stand-alone script

This lets us reuse the link checking code for the different languages
more easily.

Also: make broken links in French fail the build, it used to warn but
continue.
docs: check links in ini_config.html too

Currently fails (broken links), fix in next commit.
docs: fix a broken link in ini_config.html

At some point in the past, the anchor named "[TRAJ]-section" was renamed
to "traj-section", but one of the links to that anchor was missed
resulting in a broken link.  This commit fixes the broken link.

Also, for consistency with the other nomenclature in this file, this
commit capitalizes the ini-file section name in the document anchor, so
"traj-section" becomes "TRAJ-section".
docs: check links in all config docs

No broken links found!
add missing A axis to GEOMETRY