Add Spanish translation of mkdocs documentation

Changes

Adds a complete Spanish translation of the Domain module mkdocs documentation (10 .es.md files).

• Overview (index.es.md)
• Domain — negotiation, URL rewriting, Drush commands (domain/index.es.md)
• Path prefix (domain/path_prefix.es.md)
• Domain Access (domain_access/index.es.md)
• Domain Alias (domain_alias/index.es.md)
• Domain Config (domain_config/index.es.md)
• Domain Config UI (domain_config_ui/index.es.md)
• Domain Content (domain_content/index.es.md)
• Domain Source (domain_source/index.es.md)
• Related contrib modules (contrib.es.md)

Updates mkdocs.yml to register the es (Español) locale with its translated navigation.

Technical terms (class names, service IDs, config keys, Drupal/Drush/Symfony terms) are kept in English — only prose descriptions are translated.