Redis Analyzer isn't documented

[diamonwiggins]

Describe the rationale for the suggested feature.

There's a Redis collector/analyzer that has been around for a while, but it looks like the analyzer was never documented.

https://github.com/replicatedhq/troubleshoot/blob/main/pkg/analyze/redis.go

Describe the feature

Document the Redis Analyzer

Describe alternatives you've considered

Describe alternative solutions here. Include any workarounds you've considered.

Additional context

Add additional context about the feature request. If the change is substantial, consider attaching files to the issue outlining architectural changes, data flows, file formats etc., anything that helps describe the requested change.