Pull documentation through into docs.silverstripe.org

[GuySartorelli]

Elemental blocks are effectively a first-class citizen in the Silverstripe CMS now - 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  • Main focus is outdated docs, areas where callout blocks should be added, and updating URLs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 Move documentation linting to its own repository .github#206)

PRs

• DOC Linting #1163
• ENH Add elemental docs doc.silverstripe.org#290
• ENH Allow passing in a relative path as an argument documentation-lint#2
• FIX Additional check dirs are not the same documentation-lint#3

[GuySartorelli]

PRs merged, reassigning to Steve to check it looks correct on the site