Somewhat revive `search-assistant` with gi-docgen generted docs

The assistant mode relies on how gtk-doc generates the documentation. With more projects picking up gi-docgen it became less useful as it shows an empty window for gi-docgen based library documentation. This is an attempt to bring it back to life. We first split out the parsing into a separate function, then add some code to handle gi-docgen and do a first stab at css (which could also move to a separate file if we want to pursue that road).

This is how it looks now:

And that's before the changes in this MR:

There's a lot to be desired but it helps a lot over here so I though I'd put it out.