Publish Earthly Targets/UDCs for project documentation + mdlint/spell UDCs

Adds Earthly targets for uniformly building and testing developer documentation in a repo.

It also includes Repository Hygiene checks:

• cspell
• mdlint

These checks are not intended to be used inside a container build and are designed to be run directly on a repo.
They aim to check minimum quality standards for documentation of human-readable content in the repo.