docs: document catalog: protocol for workspace dependencies (2.8)

[lunadogbot]

Summary

Documents the new catalog: protocol shipping in Deno 2.8 (denoland/deno#32947). Catalogs let a workspace root declare shared dependency versions in one place; members reference them with \"pkg\": \"catalog:\" or \"pkg\": \"catalog:<name>\".

• Adds a new "Centralized dependency versions with catalog:" section to runtime/fundamentals/workspaces.md.
• Covers the default catalog, named catalogs (catalogs), and catalogs declared in package.json.
• Notes the precedence rule when both deno.json and package.json define catalogs (package.json wins, no merging) and the root-only restriction.

Test plan

• deno task serve — workspaces page renders, new section sits cleanly between "Sharing and managing dependencies" and "Using workspace protocol in package.json".
• deno fmt — clean.

[fibibot]

1. package.json (or deno.json) overstates where catalog: can be used. denoland/deno#32947 adds PackageJsonDepValue::Catalog, and the config schema describes catalogs as being for workspace member package.json files; catalog: is not an import-map / deno.json dependency protocol. Please remove (or deno.json) or explicitly say only package.json dependencies reference catalogs, while root deno.json / package.json can declare them.

• nit: last_modified: 2025-11-04 should be bumped for this content addition.