Add better documentation around recipe support in pyproject.toml

[Markichu]

Pull request #345 added support for adding the recipe.yaml specifications in the pyproject.toml instead of within a seperate recipe.yaml.

It would be great to see some documentation around this support. I think the Experimental Features section may be a good fit or potentially its own section with translations between recipe.yaml <-> pyproject.toml definitions.

I think this would be a great first issue for any newcomers looking to get started. This should only require changes to recipe_spec.md.