Add cross references to documentation

[kdave]

The RST format allows easy clickable cross references and currently there are almost none, the reference is done by text description of the section instead.

[kdave]

Manual pages are now linked. What's remaining:

• for command references like btrfs filesystem show we'd like to point directly to the section but the anchors must be done automatically
• when there are several references of the same command on one page, link only the first and make sure there's a link in "see also" section

[kdave]

All feature pages should link to other relevant pages like mount options, compatibility etc

[kdave]

Partial progress.

[kdave]

Duplicate labels now work for included documents, need special reference.