- use pystencils 2.0 intersphinx
 - add doc for composer builders, SfgException, config subobjects

- Move `SfgVar` to `lang`
- Remove builder for `init`, use nested function instead
- Add various docstrings

 - Add `Ref` type
 - Allow multi-arg `init` in constructor builder
 - Change `CustomGenerator` to take a composer instead of a context.
 - Allow a class to have multiple methods with the same name.