docs: Add a Redis connection page

[abdulawalarif]

Summary

• Add a Redis concept page that mirrors the Postgres connection docs: enable Redis, set host/port/password, start the development Docker instance, and connect a GUI such as RedisInsight.
• Document the correct passwords.yaml nesting (development.redis, not under database), the 8091 vs 6379 port mapping, and that the default Docker Redis has no username.
• Link the new page from Configuration, Caching, Server events, Health checks, Running your server, the configuration reference, and the Cloud Redis guide.

Fixes serverpod/serverpod#1848

Test plan

• Open Concepts → Server fundamentals → Redis and confirm the page renders, including YAML/bash blocks and tables.
• Click through Related links and inbound links from Configuration, Caching, Server events, Health checks, and the Cloud Redis guide.
• Confirm there are no em dashes and that in-docs links omit numbered folder prefixes.