Contribution description

Alternative to #21320. Quoting from there:

#21170 added documentation about RIOT_FILE_RELATIVE and RIOT_FILE_NOPATH which were already considered obsolete in #18936.

@miri64 insisted on going through the normal deprecation process even for such previously non-publicly documented macros, so lets do that instead.

There are replacements for both macros as compiler built-ins from gcc 8/12 and clang 10/9.

Testing procedure

Read the CI generated docs. Also check list of deprecated features.

Issues/PRs references

#4053, #21320, #21170, #18936