docs: Convert unshare.txt to RST and add to the user-space API manual

This is a straightforward conversion, without any real textual changes.
Since this document has seen no substantive changes since its addition in
2006, some such changes are probably warranted.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/userspace-api/index.rst

@@ -16,6 +16,8 @@ place where this information is gathered.
 .. toctree::
    :maxdepth: 2
 
+   unshare
+
 .. only::  subproject and html
 
    Indices