docs: improve index.html generation

docs: improve index.html generation

Include hypercall documentation, fixing link generation for top level links to
use the INDEX.

Allow subsection links to be renamedi in the INDEX too.

Strip .txt suffixes as well as .html ones by moving the regex to the right
place instead of placing the literal text "(?:html|txt)" into the backlink to
the top level page. (oops)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>