Option to allow Rapidoc to render unused tag descriptions

[sanscontext]

Per a conversation with Adrian on the Discord:

Currently Rapidoc "culls" (does not render) a tag descriptions if the tag is not applied to any endpoints. Other display systems allow you to do this, which allows markdown/prose API docs to render in the same display system as the OpenAPI docs. This is great for when you have prose docs that apply to many, but not all endpoints.

It looks like this is a fairly simple filtering operation, and it would be great to make this configurable.

[jmlothian]

Seconded - this is the single feature left that I need to recommend and use this project. It would enable my teams to build out docs for SDKs and create user-guides that work through multiple API calls and describe how everything fits together in a functional way. We have a lot of code-generated documentation and guides already in markdown, we just need somewhere to put them that wraps in the API docs in a single site.

I was getting ready to fork this repository to add the feature myself when I found this issue. If there's a good starting place to look at or an existing feature to leverage, I can get there a lot faster.

[wparad]

I don't know if this is relevant, but we really liked this idea in our tracking fork, so we've pulled that feature in. We're already using it for our own API