More CI checks and clean-up

[m-mohr]

Related Issue(s): #8

Proposed Changes:

• Validation for OpenAPI files (npm run check-openapi)
• Linting for Markdown files (npm run check-markdown)
• Update CI to do all checks automatically (npm run check)
• Fix issues found in OpenAPI files (see Fixes and CI for OpenAPI files  #11)
• Fix issues found in Markdown files

This doesn't enable CI check for the OpenAPI files yet as it will take us a bit more work to get our OpenAPI files valid (at the moment - includes released 0.9.0 version - they are invalid). Will make a separate PR for API changes.

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• API only: I have run npm run generate-all to update the generated OpenAPI files.

[cholmes]

Looks great, thanks for all this work.