Tidy up installation docs

Demote URL rewriting as the development server should be enough to get
people started and moving along.
Inline rewriting docs.

Fewer longer chapters generally makes the docs easier to scan and less
navigation/heirarchy to wade through.
Rename getting-started and delete fluff.
Start de-duping content in the tutorial.

Having 2 copies of all the installation info makes the docs really
verbose and much harder to maintain. Having fewer duplicate sections
will also help readers as they have less stuff to go through.
Merge pull request #1773 from cakephp/update-installation

Update installation
Fix broken/missing links.
Reduce heading levels.

This removes the tutorials from the TOC which is nice as they are
inlined elsewhere.
Make the dropdown wider.

It looked stupid before.