docs: #3588137 Cross-link domain_config_ui docs to domain_menu_extras submodule

Summary

Adds a Related submodules cross-reference at the end of docs/domain_config_ui/index.md (and the FR/ES siblings) pointing readers at the new domain_menu_extras submodule in the domain_extras companion project.

Discoverability follow-up to #3588057 (closed as won't fix) and #3588108 (where the actual fix lives).

Why

Per-domain configuration can affect rendering that depends on language-keyed plugin definitions cached by Drupal core (LocalTaskManager, LocalActionManager, ContextualLinkManager). Without a cache key that also varies by domain, the first domain to warm the cache wins and subsequent domains render stale tabs and actions. The fix is scoped to the domain_menu_extras submodule so sites that don't override per-domain config affecting menu task definitions don't pay the cache fragmentation cost; but a reader of domain_config_ui had no signpost pointing at it.

Notes

• One paragraph per language; ~70 chars per line; cspell clean (FR/ES are in _CSPELL_IGNORE_PATHS anyway).
• Technical terms (local tasks, local actions, contextual links, cache key, plugin managers, domain-aware) stay in English in the FR/ES versions; the prose is reformulated rather than translated word-for-word.

Closes #3588137