docs: link generated API documentation

[fengyangxxx]

This change replaces the legacy godoc.org documentation link with a
project-named Read the Docs site generated from Redsync source by Sourcey:

https://redsync-sourcey-docs.readthedocs.io/en/latest/

The initial site is pinned to Redsync commit
79f6ba24a8bf41f35141de700d410a06bb27622f. It covers the root locking API,
Redis interfaces, and the go-redis, Redigo, Rueidis, and Valkey adapter
packages, with search and links back to the corresponding source lines.

The reproducible build source and generated static output are maintained at:

https://github.com/fengyangxxx/redsync-sourcey-docs

The documentation project is transferable. I can invite Redsync maintainers
to both the Read the Docs project and the backing repository so the project
can own future rebuilds or move the static output to a project-controlled
host.

Verification performed before opening this PR:

• Source commit: 79f6ba24a8bf41f35141de700d410a06bb27622f
• Sourcey version: 3.6.3
• Generated coverage: 21 pages, including 15 API package pages
• Inventory: 15 packages, 19 non-test Go files, 110 exported symbols
• Five page-to-source mappings checked against the pinned commit
• Root compile-only check and focused redis/... tests passed
• Full go test ./... requires the external redis-server binary, which is
  not installed in the documentation build environment

No library code or runtime dependency changes are included.