Expose types and functions of package

[l0nax]

I'm very grateful for this project, and I really like it. Thanks for all your effort :)

The reason behind this PR is that I want a more pkg.go.dev like experience, i.e. a sidebar/ navbar that updates based on the position on the page, etc.

[abhinav]

Thanks for the PR, @l0nax. It makes sense to expose this information in the front matter data—I assume you're using this information somehow to fill a TOC?

That said, I don't think the raw godoc.Types should be exposed.
Those include a lot of the information needed to render the HTML, and don't necessarily have a stable shape.
For example, none of the decl-rendering or comment rendering logic will work in the frontmatter template.

If we could instead create a copy of a subset of that information for frontmatterData, that would be much better.
For example, for the types, we would probably have:

type frontmatterType struct {
  Name string

  Constants, Variables []*frontmatterValue
  // ..
}

type frontmatterValue struct {
  Names []string
}

It'll need a little boilerplate to build this mirror subset, but it'll be a stable API for the frontmatter templates that I can commit to not breaking in the future.