docs(sourcey): add documentation generator

[auscaster]

Adds the current Sourcey documentation engine to the generator catalog. It is an AGPL-3.0-only TypeScript project that builds static HTML from OpenAPI, MCP, Doxygen, godoc, rustdoc, and Markdown sources.

Product page: https://sourcey.com/docs
Repository: https://github.com/sourcey/sourcey

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[matbrgz]

Heads-up: there are now four open PRs adding the same src/site/generators/sourcey.md: this one, #1089, #1096 and #1097. Only one can be merged. This PR is the earliest and comes from the project's maintainer. #1089 has the most complete and current description (OpenAPI, MCP, Doxygen, godoc, rustdoc, MkDocs, static output).

If this PR is the one kept, a few fixes:

• templates: TypeScript isn't a template language. Markdown (and MDX if supported) fits better.
• language: the repo is TypeScript, not JavaScript.
• The description could cover the newer inputs (MCP, Doxygen, godoc, rustdoc) that the other PRs mention.