docs: Setting up Sphinx documentation

[ns-rse]

Closes #14

• Sets up Sphinx documentation under docs/source.
• Skeleton documents written in Markdown in place for...
  • introduction.md
  • installation.md
  • usage.md
  • contributing.md
  • workflow.md
• Will build API documentation automatically via Sphinx AutoAPI.
• Configuration is via docs/source/conf.py.
• Uses the pydata sphinx theme.
• Builds multiple versions based on tags beginning with v* and main using Sphinx Multiversion

[ns-rse]

Currently the GitHub Action which builds the documentation and deploys to GitHub pages fails because the cgat package requires Numpy to be installed first but for some reason hasn't pulled it in. Manually installing Numpy doesn't seem to solve this (see issue reported here).