Add wiki page on making a new entity type translatable

[Rohan-Unbeg]

Overview

This PR adds a guide for extending Oppia's translation infrastructure to a new
entity type. It comes out of the work in oppia/oppia#24933, which generalised
translations from explorations only to skills and exploration metadata.

It covers:

• The four parts of the translation system that a new entity type has to be
  registered with.
• Making a domain object translatable, and giving every field a stable content ID.
• The opportunity lifecycle, including the difference between an entity that stops
  being reachable and one that is deleted.
• Suggestions, serving translations to learners with an English fallback, and the
  contributor dashboard endpoints.
• Backfilling existing entities, making translated content searchable, and rolling
  out behind a feature flag.
• A checklist, and the gotchas that cost the most time while building this.

What changed

• Added Adding-a-new-translatable-entity-type.md.
• Updated _Sidebar.md to link it under the I18n/Translations Guide entry.