Overhaul docs and makefile

Overhaul the underlying structure of the documentation.

* Expand `docs/index.rst` with information taken from `README.rst`,
  `docs/installation.rst` and `docs/getting_started.rst`.
* Make `README.rst` a link pointing to `docs/index.rst`.
* Drop `docs/installation.rst` and `docs/getting_started.rst`.

Why has this been done?

* Making `README.rst` a link means that there is now a single, authoritative
  source for documentation. Formerly, `README.rst` and the various `docs/*`
  files contained much duplicate information.
* Merging `docs/installation.rst` and `docs/getting_started.rst` into
  `docs/index.rst` is simply sensible. The documents are short, and it is
  reasonable for their information to be contained in one page.

Update the makefile with a more robust, expansive and consistently named set of
rules.

* The rules are more robust because they use `&&` instead of `;` for compound
  commands.
* The rules are more expansive because a new rule has been added
  (`docs-doctest`) and the doctests are run when the `test` rule is evaulated.
* The rules are more consistently named because the prefix "docs-" is used to
  denote all rules defined in `docs/Makefile`.

Fix broken doctests. It is now easy to test this because `make test` runs both
normal unit tests and doctests.