Most of the samples were already 4-space indented, but a few were using 2-space,
which is both inconsistent and, when it happened with Python code samples,
incompatible with PEP8.
All `.. code::` instances have been changed to use `.. code-block::`,
and have been properly formatted. The syntax names have been normalised,
so all django templates use the `html+django` syntax, shell commands use
`sh`, and plain text uses `text`.
