docs: Help on creating filters and showing them in the UI

Hi!

I'm looking for more information on how to create filters (I think that's what I need?) and how to display them inside Docusaurus specifically.
An example of what I mean is available on the [Readme website](https://docs.readme.com/docs), where we can search by docs category.

The filters docs on the Algolia website seem to only mention how to create them in the index but the end of the story seems missing: how to display them to users.

I'm happy to open a PR here to add a tutorial. I think it's a high value tutorial for most docs websites using your product 👍

Start by reviewing the existing filters documentation on the Algolia website and the Docusaurus-specific search setup described in the issue. Document how to create filters and display them to users, with a tutorial example that demonstrates filtering docs by category.

Written by the indexing model from the issue text.