Add homepage and source URL to library Libdoc documentation

Problem

Libdoc shows a library's version, scope and keywords, but there is no standard place to link back to the project — where to install it from, report issues, or contribute. Library docs are often published separately from the source repo and package, so users who land on generated docs have no way back.

Proposal

Add fields for an official library URL homepage and source that Libdoc renders (HTML):

• A ROBOT_LIBRARY_URL class/module attribute, mirroring ROBOT_LIBRARY_SCOPE / ROBOT_LIBRARY_VERSION.
• A matching @library(url="https://...") argument, like the existing scope, version, doc_format, listener.

@library(scope="SUITE", version="1.0", url="https://github.com/example/MyLibrary")
class MyLibrary:
    """My example library."""

Alternatives

• Free-form metadata dict (e.g. ROBOT_LIBRARY_METADATA = {"Homepage": "..."}) — more flexible but heavier than one well-known field.
• Docstring convention only — works today but is unstructured and inconsistent between libraries.

Open questions

• Attribute name: URL vs HOMEPAGE vs SOURCE; one URL or a small set of named links.

Related issues

• #5637 Support specifying suite, test and keyword metadata as custom settings (open, high) — a library-level URL could fit the same metadata mechanism.
• #5627 Keyword metadata (open, high) — similar direction, keyword-scoped.