Migrate docs

[stephwills]

This PR migrates the docs to use mkdocs instead of Jekyll, bringing it in line with the Paricia documentation.

Summary of changes:

• The docs follow a similar structure to Paricia, i.e. the docs folder now contains the markdown files in subdirectories en and es, and the config folder contains the mkdocs.yml files for configuration
• overrides/assets contains images and extra css
• This overwrites the existing docs directory for the jekyll site (hence large number of files deleted)
• I've created workflow files to test the docs build as part of CI and to deploy docs when there is a push to main
  • The deploy-docs workflow should run mkdocs build for each language config, and then deploys the build site dir to a gh-pages branch
  • Note: I've created an index.html file in the root directory, which gets copied into the built sites dir -- this just ensures the index page redirects to the English version of the index as default
  • This will need setting up in Settings/pages (which only admin has access to)

To test locally install requirements.txt and run mkdocs serve config/en/mkdocs.yml or mkdocs serve config/es/mkdocs.yml, then navitage to http://127.0.0.1:8000/riverlabs/.