Add RDoc documentation site for Rubydex APIs

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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)

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

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