Documentation: attempt consistency in headings and links

This changes the capitalization of headings and aims to get a consistent
style of internal links. Linking to chapter numbers is very fragile as
asciidoc sadly doesn't support this in an automated fashion - so instead
we reference chapter titles (also manual, but at least less fragile when
chapters are inserted)

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>