Table of contents for individual tutorials

[simonw]

e.g. https://datasette.io/tutorials/explore - I want a table of contents at the top of the page based on the document headers, similar to the one on https://docs.datasette.io/en/stable/plugin_hooks.html

[simonw]

I'm going to implement this with a custom template function that uses BeautifulSoup (aready a dependency for datasette-render-markdown) to add both the ToC and the id="x" attributes to the nested headers on the page.