feat(docs): submenu open for h3 in docs page

[elpiel]

Here's a useful (to me) idea to allow seeing built-ins (filters, fns, etc.) directly in the menu.
One way this could be improved is to remove some headings in the docs (For, Filters):

here's a view of the Built-ins menu:

[Keats]

Wouldn't it make the menu too big in practice when opening the filters? Since the docs are a single page I don't think it's a huge pain point to search?

[elpiel]

The reason I was checking the documentation in the first place was not so much to see what each filter does, but rather to remind myself of the possible filters.

That's why I said useful for me 😅