Modify the installation instructions

These changes are designed to make the installation instructions
clearer. Key features are:

* Dependencies gathered together.
* Hints which turn into links in HTML to help users skip to the right
  bits.
* Fewer installation commands for each user.
* Rejigged heading structure to make it clear which parts belong together.
[GitHub doesn't interpret](https://github.com/github/markup/issues/122) the `:ref:` role, since it's a Sphinx role. Doesn't really matter though.