About: provide module-level doc
About: provide git commit if possible
api/index.html: provide version, git commit, and date

Also renamed intro.txt[.in] to index.txt[.in]. Didn’t care to do this,
but for some mysterious reason, with the index.txt filename, `omake
doc` caused the generated intro.txt to go into src/doc in addition to
_build/src/doc.
doc/api: move files directly here

We had them elsewhere and then copied. Now putting directly in the
hierarchy that we want.