and use it on templates.md

manfilter.lua learns how to turn a link to another document in docs/
into a manual page cross-reference. This is paralleled by a
pandoc-website filter change for handling such links.

a lightly customized template for turning arbitrary docs into manual
pages is provided, which exploits the standard `subtitle` metadata for
the NAME section of the manual

This first stab at this work does the brutal minimum amount of work
while still being somewhat extensible. The new Makefile rule that
applies the new man template hardcodes section 5. The mapping of
document titles to manpage names and sections used by the lua filter is
hardcoded at the top. Cleverer ideas could reduce repetition, at the
cost of having to read yaml frontmatter out of all the docs to support
cross-references, or relying on exotic GNU Make tricks, or similar.

Some unrelated changes picked up in pandoc.1 since that's usually a
release task.

UNTESTED