Closes #6751

todo:
- Subclass SphinxDirective instead of Directive

recipe:
- Remove unnecessary '__init__' methods

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

helloworld:
- Return version metadata from extension
- Use 'reST' instead of 'rST'
- Don't use single backticks

todo:
- Return version metadata from extension
- Link to events section of API guide, rather than entire document
- Include name of phases in when describing build phases
- Use more method cross-references where possible
- Fix typo in usage example

recipe:
- Return version metadata from extension
- Rework code to simplify things
- Remove docstrings from 'generate' functions, which are simply
  duplicates of the original docstring
- Rename 'rcp' directive to 'recipe', the 'reref' role to 'ref', and a
  whole lot of variables to something more grokable
- Fix typos in both documentation and code

I've also fixed up the docstrings for some of the 'domain' functions to
make them render a little nicer (they took me a while to figure out).

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

This avoid duplication and could conceivably let us test this stuff in
code later on.

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

Adopt the same format as was recently added in the "hello world"
tutorial, making this more of a walkthrough as expected from tutorials.

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