allow toctree with titles_only=False

[wojdyr]

I switched from the RTD theme to Furo primarily for dark mode.
RTD allows setting toctree parameters through html_theme_options:

Furo, as I understand from #354, is opinionated and intentionally limits customization.

Still, I think adding parameters titles_only and maxdepth for the toctree call would be helpful for some docs and wouldn't harm others. First, let me illustrate. I monkey-patched Furo to get one level of headers in pages:
https://github.com/project-gemmi/gemmi/blob/f11d1c4d84843e621770921bfdc88c39ce1ec2fb/docs/conf.py#L47-L69

This gives me sidebars like this:

Having one level of in-page sections shown on the left helps me:

1. when the screen size is such that the responsive design shows only the left side sidebar
2. when I'm not wearing glasses – the right sidebar is harder to read
3. when I want to check what sections a different page has without leaving the current page.

Consider adding titles_only + maxdepth to the supported configuration.