Pull documentation through into docs.silverstripe.org

userforms is supported and commonly used. The documentation should be on the docs.silverstripe.org site.

Acceptance criteria

[ ] Documentation is (briefly) reviewed and improved or updated where necessary
- Main focus is outdated docs, areas where callout blocks should be added, and updating URLs
- Restructuring the docs is at the implementer's discretion
- URLs to other dev docs should be relative paths for docs in this repo, and https://docs.silverstripe.org/en/developer_guides/model/data_model_and_orm/ style (note no version number) for other docs
- API links must use [ClassName](api:FQCN) format
[ ] Dev docs sources-docs.js is updated to pull in docs from this site
[ ] userguide is not pulled into dev docs, but is still pulled into userhelp docs
[ ] Apply linting to documentation (see https://github.com/silverstripe/.github/issues/206)