In Python 3, the default encoding of source files is utf-8. The encoding
cookie is now unnecessary and redundant so remove it. For more details,
see the docs:

https://docs.python.org/3/howto/unicode.html#the-string-type

> The default encoding for Python source code is UTF-8, so you can
> simply include a Unicode character in a string literal ...

Includes a fix for the flake8 header checks to stop expecting an
encoding cookie.

This one is a little unusual. The current theming doc mixes user-focused
documentation with developer-focused documentation. This is confusing
and unnecessary. Resolve the issue by splitting the doc in two. The
user-focused docs are moved to the advanced section of the usage guide,
while the developer docs remain where they are, pending the addition of
a developer guide.

Signed-off-by: Stephen Finucane <stephen@that.guru>

This is added to the new "advanced" subsection.

Signed-off-by: Stephen Finucane <stephen@that.guru>

This is simply the former 'tutorial' document renamed. A number of
references need to be updated, so this is done.

Signed-off-by: Stephen Finucane <stephen@that.guru>

I'm not entirely sure what this line is trying to say, but is needs rewriting. Modify my commit, if necessary.