ENH - Allow setting show_nav_level per page

[simonspa]

On our page we would like to collapse the sidebar TOCs down to their :caption: elements as described in the PyData Theme docs.

However, we would like to do this only on a single page. Currently this can only be set as a global theme option.

This PR adds the possibility of setting this per-page via the page metadata as described in the docs:

---
html_theme.show_nav_level: 0
---