With the exception of sbcl.texinfo, backmatter.texinfo, and
asdf.texinfo, all other Texinfo files are from
SB-MANUAL::GENERATE-TEXINFO.

With the exception of variables.texinfo, the generated files are under
version control, to keep a closer eye on the Markdown-to-Texinfo
converter, which is young. This is redundant, of course. In the
fullness of time, in due course, when conditions allow, at the
appropriate juncture, we may want to revisit this decision.
