Add Create a backend add-on section in the Developer guide.

[stevepiercy]

• Enhance the Development Guide index
• Update the term Buildout in the Glossary to point to dusty old, yet useful, docs in Plone 4.

See https://github.com/plone/documentation/pull/1748#discussion_r1827660310

Closes #1754.

Ping @petschki to take it from here.

---

📚 Documentation preview 📚: https://plone6--1757.org.readthedocs.build/