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>

Documentation/sphinx/automarkup.py

@@ -51,7 +51,7 @@ RE_typedef = re.compile(r'\b(typedef)\s+([a-zA-Z_]\w+)', flags=ascii_p3)
 # Detects a reference to a documentation page of the form Documentation/... with
 # an optional extension
 #
-RE_doc = re.compile(r'\bDocumentation(/[\w\-_/]+)(\.\w+)*')
+RE_doc = re.compile(r'(\bDocumentation/)?((\.\./)*[\w\-/]+)\.(rst|txt)')
 
 RE_namespace = re.compile(r'^\s*..\s*c:namespace::\s*(\S+)\s*$')
 
@@ -234,7 +234,10 @@ def markup_doc_ref(docname, app, match):
     #
     # Go through the dance of getting an xref out of the std domain
     #
-    target = match.group(1)
+    absolute = match.group(1)
+    target = match.group(2)
+    if absolute:
+       target = "/" + target
     xref = None
     pxref = addnodes.pending_xref('', refdomain = 'std', reftype = 'doc',
                                   reftarget = target, modname = None,