add `.. file::` directive

Is your feature request related to a problem? Please describe.
Like environment variables, I want to have a specific page in my user manual to explain all files relevant to my application (like you usually have a FILES section in a manpage).

Describe the solution you'd like
.. file:: would make sense, similar to .. envvar::. Then when you do

:file:`myfile`

, if there is a .. file:: myfile entry somewhere, it will link to it.

Describe alternatives you've considered
There's probably a way to implement a custom directive, but since we already have program, option and envvar, I think it would make sense to have file too.

Additional context
See our environment variables page for example: https://manual.zrythm.org/en/appendix/environment.html

In the next page, I want to have something similar for "Files and Directories"

Start by examining Sphinx's existing envvar, program, and option directive and role implementations, along with the linked environment-variables page. Define the file directive and :file: role to mirror their behavior, then verify that a documented file entry resolves to a link from :file:myfile.