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.
For example, a link to language.html#number is converted to point to
https://elv.sh/ref/language.html#number instead. Such links are quite common in
elvdocs.
Also convert all link destinations that use ./language.html to use language.html
instead, and add a check in tools/check-disallowed.sh to ensure that remains the
case.
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.
