docs: Enable usage of relative paths to docs on automarkup

Previously, a cross-reference to another document could only be created
by writing the full path to the document starting from the
Documentation/ directory.

Extend this to also allow relative paths to be used. A relative path
would be just the path, like ../filename.rst, while the absolute path
still needs to start from Documentation, like Documentation/filename.rst.

As part of this change, the .rst extension is now required for both
types of paths, since not requiring it would cause the regex to be too
generic.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Link: https://lore.kernel.org/r/20210128010028.58541-2-nfraprado@protonmail.com
[jc: Tweaked the regex to recognize .txt too]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>