[Diagnostics] Print category footnotes at the end of translation
When printing diagnostics, category names are printed as [#<category-name>]
at the end of a diagnostic. For all of the category names that are mentioned
in this manner, print "footnotes" at the end of compilation providing
documentation references to each category, e.g.,
[#deprecated]: <http://example.com/deprecated>
[#StrictMemorySafety]: <http://example.com/memory-safety>
Right now, these point into the markdown files in the installed toolchain,
same as the URLs behind references. That is subject to change in the future.
