PHPLIB-523, PHPLIB-524: Restructure docs index and install pages

[jmikola]

https://jira.mongodb.org/browse/PHPLIB-523
https://jira.mongodb.org/browse/PHPLIB-524

A third commit also fixes various syntax/formatting issues I noticed while working on these two pages.

Opening against v1.1 as I can apply there and merge up to master so this changes applies to all published versions of the manual. Separately, I'd propose we consider hiding some older version in the drop-down. I can't imagine that docs for v1.1 is very relevant for anyone.

To build locally from a clone of https://github.com/mongodb/docs-php-library with this repo cloned in ../phplib:

#!/bin/sh

rm -r build/master
rsync -a --delete ../phplib/docs/ source/
giza -l warning make publish

You can then serve the docs with:

#!/bin/sh

php -S localhost:8080 -t ./build/master/html/