This adds a workflow to check that the site builds in strict mode, which should normally be the case. The main purpose of this is to catch internal link fails on PRs.

• adds a workflow
• mandates 1.61 version of mkdocs
• It needs it's own mkdocs.yml file (mkdocs-test.yml) so that:
  • it can build without the material-preview
  • it can save time by cutting out some other things which increase build time but won't result in link errors etc.
• to facilitate above and to prevent drift, this pr also separates the nav from the main mkdocs.yml

the workflow will remain silent unless the strict build fails, in which case it dumps the mkdocs output into a comment and marks the workflow as failed