Build docs with mkdocs, publish to gh-pages

[spwoodcock]

• Updated the docs dir.
• Needs to be built with mkdocs-material (if building manually).
  • I didn't mess with the requirement files - we can update to use a dep manager like pdm in the future.
• Added workflow to auto build docs and push to gh-pages on push to master.

Note: after first auto-build, you may need to go to Settings --> Github Pages --> Set branch to gh-pages to deploy from. The final url should be: https://hotosm.github.io/osm-export-tool