For example, [`put`]() links to the elvdoc for "put".
Also remove the @cf macro now that implicit targets make it easier to link to
other elvdoc sections.
We are no longer using Pandoc, so this limitation can be lifted.
Also make @cf generate internal links where applicable in non-builtin modules:
"@cf mod:fn" in the doc of "mod" now links to just "#mod:fn" instead of
"mod.html#mod:fn".
The elvdocs still use the old format (#elvdoc:fn or #elvdoc:var) for now, but
will be changed to "fn" and "var" forms soon.
Also remove the accidentally committed cmd/mvelvdoc. It has been used to perform
the conversion automatically but is not supposed to be committed.
