Fetch package docs from JSR, NuGet, and Docker Hub APIs

[christian-andersson]

Summary

• get_package_docs now fetches documentation directly from JSR, NuGet, and Docker Hub APIs instead of skipping them and going straight to the GitHub README fallback.
• All three registries expose description/README content in their public APIs — we just weren't using it.
• Registries covered by native API docs goes from 6 to 9 (out of 13).

Implementation notes

• JSR — description from /scopes/{scope}/packages/{name}
• NuGet — description from the latest catalog entry in registration5-gz-semver2
• Docker Hub — full_description (falls back to short description) from /v2/repositories/{name}
• GitHub README fallback still applies when the registry returns no content.
• Maven, Go, Swift, and GitHub Actions continue to fall back to GitHub since their APIs don't expose descriptions.

Test plan

• deno fmt --check && deno lint && deno check main.ts && deno test --allow-net all pass (verified locally)
• Call get_package_docs for jsr:@std/path and confirm source: "registry" with JSR description
• Call get_package_docs for nuget:Newtonsoft.Json and confirm source: "registry" with NuGet description
• Call get_package_docs for docker:nginx and confirm source: "registry" with Docker Hub full description
• Call get_package_docs for a package with no API description (e.g. docker: of a repo without full_description) and confirm GitHub fallback still runs

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