Extend documentation

[chriscarrollsmith]

• Add another graph and some explainer text to document auth flow architecture
• Optional: Add _quarto.yml and render docs as a multi-page HTML website (to be served over Github pages) and link these docs from the README and the main page of the template (then simplify the README)

For an example of how to generate architecture graphs using graphviz, take a look at the existing graph generation code in README.qmd.