Update and organise the contents page for easier reference.
Organise glossary links alphabetically.
Add more index entries.
Major re-ordering of the docs to make the index, contents and
next/prev work better.
Remove the Site link, our docs are too big for it to be sensible.
Reword some of the headings to make the contents page make more sense.
Add two new sections into the contents page: "first steps installing"
and "migrating".
Link to files in tocrees without the .rst extension, as recommended by
various docs.

Change-Id: I84eb6b903bd8ebf8852a917956f8089bb4469081