Filter documentation via a query parameter for better browser interoperability

# Feature request
Filter documentation via an optional query parameter

## Summary
DevDocs makes it really easy to filter on a specific set of docs in the UI, which is great. I use this in conjunction with browser site search to pass queries directly to the DevDocs search endpoint to skip landing on the main page first. However, there is one disadvantage: I can't seem to filter the documentation from the search endpoint directly. If want to search for a ruby-specific method from site search, I need to land on the main DevDocs page and filter manually.

[Browsers are embracing custom site search functionality](https://resources.arc.net/en/articles/7183263-site-search-directly-search-any-website) to allow quick access to search endpoints that can be invoked via a shortcut and this has become an important part of my workflow. It would be great to have documentation filtering in the search API to improve the interoperability of DevDocs search.

## Examples

The query endpoint looks like this:

`https://devdocs.io/#q=%s` where `%s` is replaced with my query string

It would be great to be able to do something like this to search only within Ruby 3.1:
`https://devdocs.io/#q=%s&doc=ruby3-1`

Start at the DevDocs query endpoint shown in the issue, https://devdocs.io/#q=%s, and trace how the browser handles the q parameter and documentation filtering. Done means a URL can include doc=ruby3-1 so the search runs only against the requested documentation set.

Written by the indexing model from the issue text.