Add link checker script
Add a link checker script and instructions on how to run it.
First run found some problems, fix those as well:
* Escape some url templates from link creation with a '\' prefix,
the templates need to be turned into valid urls
by inserting a package name.
* For some reason,
the link checker cannot follow https://crates.io/crates/bodhi-cli.
This tool is also packaged in the official Fedora repositories,
and it is customary to refer to such tools simply by tool name,
so replacing the link with that
instead of excluding that link for the check.
* https://bodhi.fedoraproject.org/updates/new requires authentication.
It is not wrong to link there,
but since the Bodhi web app main page also has a "New Update" button,
replacing the link there,
so even unauthenticated readers get a good experience.
* Fix link to nonexistent page at src.fedoraproject.org
with a link to site root,
which is suitable in the context.