Add RDoc logo to documentation

[st0012]

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On GitHub:

[matzbot]

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[Copilot]

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The new README image reference will produce a broken image when users generate docs via the plain rdoc command unless .rdoc_options (or equivalent CLI guidance) is updated to copy the asset too.

Pull request overview

This PR adds an RDoc logo asset and wires it into both the repository README and generated Aliki-theme documentation output, ensuring the image is packaged with the gem and copied into generated site output.

Changes:

• Add rdoc-logo.svg and reference it from README.md.
• Ensure the SVG is included in gem packaging and copied during RDoc generation (rdoc.gemspec, Rakefile).
• Add Aliki CSS rules to style the logo block in generated HTML.

File summaries

File	Description
README.md	Adds a centered logo image at the top of the README.
rdoc.gemspec	Packages the SVG and adds --copy-files rdoc-logo.svg to gem RDoc generation options.
rdoc-logo.svg	Introduces the new logo asset as an SVG.
Rakefile	Adds --copy-files rdoc-logo.svg to the rake rdoc task options so site builds include the asset.
lib/rdoc/generator/template/aliki/css/rdoc.css	Styles the logo block for the generated docs (including a dark-theme variant).

Copilot's findings

• Files reviewed: 4/5 changed files
• Comments generated: 1

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[st0012]

Ideally this should be under doc/. But moving it there would cause file path mismatch in generated doc (copied file is always placed on _site/, so source will still be rdoc-logo.svg) and GH (will use doc/rdoc-logo.svg). So I'll leave that to another discussion/PR.