Create redis-setup.md (#2520)

Co-authored-by: Kim Tsao <84398375+kim-tsao@users.noreply.github.com>

docs/redis-setup.md

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+# Setup Cache service with Redis Cache
+To complete this setup, you need an active Redis server. 
+A good option when deploying on Openshift is using *Redis Enterprise Operator* provided by Redis (marked as Certified, official Redis Docs [here](https://redis.io/docs/latest/operate/kubernetes/deployment/openshift/openshift-operatorhub/)), but any Redis instance should be fine; feel free to chose any deployment method you prefer.
+
+To enable the Cache service in RHDH, you need to add the following configuration to your app-config.yaml:
+
+```
+backend:
+  cache:
+    store: redis
+    connection: redis://user:pass@cache.example.com:PORT
+```
+The `useRedisSets` option is no longer supported (from RHDH 1.5).
+
+## Use Redis with plugins
+The plugins using Redis are Techdocs (production-ready) and Bulk-import (tech preview).
+
+To use Redis with Techdocs, make sure you set the ttl parameter in the Techdocs section of your configuration, or the items won't be cached:
+```
+# File: app-config.yaml
+
+techdocs:
+  # techdocs.generator is used to configure how documentation sites are generated using MkDocs.
+  .
+  .
+  .
+  cache:
+    # Represents the number of milliseconds a statically built asset should
+    # stay cached. Cache invalidation is handled automatically by the frontend,
+    # which compares the build times in cached metadata vs. canonical storage,
+    # allowing long TTLs (e.g. 1 month/year)
+    ttl: 3600000
+
+    # (Optional) The time (in milliseconds) that the TechDocs backend will wait
+    # for a cache service to respond before continuing on as though the cached
+    # object was not found (e.g. when the cache service is unavailable). The
+    # default value is 1000
+    readTimeout: 500
+```
+
+## Troubleshooting
+The logs will no show any hint of the cache service being running correctly. You can set more verbose logging by passing the enviroment variable `LOG_LEVEL=debug` to your RHDH instance. 
+You will only see error logs when a cache read is requested by any of the plugins (for example, when opening a Techdocs page).
+Some examples:
+- if you see errors like:
+  ```
+  {"level":"\u001b[31merror\u001b[39m","message":"Request failed with status 500 NOAUTH Authentication required","plugin":"techdocs","service":"backstage","span_id":"f2f0499492ded848","timestamp":"2025-02-20 13:37:03","trace_flags":"01","trace_id":"c750094ef56da1412f217c3a2cd01ded","type":"errorHandler"}
+  ```
+  or
+  ```
+  {"level":"\u001b[31merror\u001b[39m","message":"Failed to create redis cache client WRONGPASS invalid username-password pair","service":"backstage","span_id":"cfaf6a6a2031a34d","timestamp":"2025-02-20 13:27:15","trace_flags":"01","trace_id":"c0196b4be2bb94bba433290d8311d18b","type":"cacheManager"}
+  ```
+  make sure you have setup at least a password for the dafault user (or better, a proper user with the rights to access your Redis database) and that you are providing the correct values in the connection string
+- if you are a using ssl/tls (i.e. `rediss://user:pass@cache.example.com:PORT`), make sure all the certificates are in place and valid; if you are using self-signed certificates, the connection will probably fail to validate the CA; **for testing porpuses only**, you can set the `NODE_TLS_REJECT_UNAUTHORIZED=0` enviromnment variable to overcome this issue