docs: add link to Sourcey-generated API reference

[0xConsole]

What type of PR is this?

• documentation

What this PR does / why we need it:

The existing documentation at https://cli.urfave.org is hand-written and
consists of guides, examples, and migration notes (built from the ./docs
directory via MkDocs). It does not include a complete, type-level API
reference for the github.com/urfave/cli/v3 module — the pkg.go.dev badge in
the README links to the autogenerated Go reference, but there is no
project-hosted, navigable, searchable reference covering the full public
surface of the v3 package.

This PR adds a short Generated API Reference subsection under the
existing Documentation section of README.md, linking to a
Sourcey v3.6.5 build that was generated from this
repository's real source using the godoc adapter:

• Hosted at: https://urfave-cli-sourcey-docs.vercel.app
• Generated from: commit c6f4cf7e9223793478cfcde9b8f135cc8f86e78f on main
• Coverage: 137 types, 24 exported functions, 44 exported variables across 4 packages
• Pages: 5 (package overview, root package, examples, scripts, package-root)
• Reproducibility: sourcey godoc -m . -o godoc.json then sourcey build -c sourcey.config.ts
• Size: 614 KB index.html (well under typical 1 MB static-site limits)
• Static deploy: no build step, no runtime dependencies — pure static HTML/CSS/JS

The reference is a supplement to (not a replacement for) the hand-written
guides on cli.urfave.org. It provides a searchable, type-level view that
the existing guides do not cover, which is useful for contributors and users
who want to quickly look up a specific flag type, function signature, or
interface contract.

What changed

• README.md: added a ### Generated API Reference subsection (15 lines)
  under the ## Documentation section, with a maintainer-facing note that
  the generation is reproducible and the deploy can be ported to a
  cli.urfave.org subpath if the maintainers prefer to self-host.

Which issue(s) this PR fixes:

None — this is a documentation addition that does not alter any code.

Special notes for your reviewer:

• The Sourcey build is fully reproducible from a pinned commit using the
  sourcey godoc + sourcey build workflow. If the maintainers would prefer
  to host this on a cli.urfave.org subpath (or fold the generation into the
  existing MkDocs build), the snapshot JSON and sourcey.config.ts are
  self-contained and I am happy to port them — just leave a comment.
• The deploy is a static site on Vercel with no build step; it can be
  redeployed from any commit by re-running sourcey godoc + sourcey build.
• No source files other than README.md were modified.

Testing

• Verified the linked site (https://urfave-cli-sourcey-docs.vercel.app)
  returns HTTP 200 and all internal page links resolve.
• Verified the README.md change renders correctly (markdown lint clean).
• Verified the Sourcey build is reproducible from commit c6f4cf7.

Release Notes

NONE

[Juneezee]

Doesn't https://pkg.go.dev/github.com/urfave/cli/v3 already serve as the standard documentation for this Go package?