Add RDoc documentation site for Rubydex APIs

[st0012]

Problems

• Rubydex does not have generated API documentation that can be published as a static site.
• The native extension exposes Ruby APIs from C, but most public C-backed methods are not documented in a format RDoc can capture.

Solutions

• Add RDoc configuration, a documentation rake task, and a GitHub Pages workflow using the current RDoc commit.
• Include both Ruby and C extension sources in the generated site.
• Convert public C-backed API comments to RDoc call-seq: blocks with RBS-ish return shapes.

(The workspace_path attribute's type label is weird but it's a RDoc issue)

[st0012]

RDoc's C parser only recognizes this style of C comments.

[st0012]

RDoc currently doesn't have a way to read RBS sigs from C files yet. So let's use call-seq for now.

[vinistock]

Doesn't GH pages deploy automatically if docs is modified? Maybe something changed, but I remembered the process being a bit simpler.

For example, the Ruby LSP GH pages website just builds everything in docs and pushes a commit to a specific branch.

[st0012]

That's the branch-based deployment approach. I chose a different approach here as explained in #874 (comment)

[vinistock]

After this, we really ought to give some love to the architecture docs, which are really behind and lacking at the moment. I ended up never finishing #423

[vinistock]

It's not the best that we need to spread this in almost all of our C files. Is there already a solution in RDoc? If there is, let's create a ticket to get rid of these so we don't forget.

[st0012]

I asked Codex to prototype a solution but I haven't looked into it yet.

[st0012]

#885