Adding documentation for new Redis configuration variables

[scottisloud]

This PR resolves issue #278 .

• Documents the newly available environment variables to configure redis for custom redis deployments.
• Adds placeholders or commented-out variables in configuration manifests where possible.
• Leaves OP_REDIS_URL as the default in all configurations.

The introduction of these new environment variables in #276 and their documentation in this PR, provide customers with more flexible options for custom redis implementations, especially around handling authenticated redis connections.

[scottisloud]

I think I've done a preliminary update to the requisite README files and .env for all the deployments except:

• AWS deployments, which will need a bit more work to determine the best way to document these and optionally include them in the config manifests.
• The Azure Container Apps and Digital App Platform deployments where I doubt these advanced options will ever get used and where simpler may be better. I'm open to revisiting this one though if opinions differ.

[scottisloud]

I've added documentation for the new environment variables in all READMEs where relevant, and provided commented-out variables in configuration files where appropriate (e.g., not currently possible in Cloud formation, either not possible or deeply unlikely in DO and Container Apps).

Largely looking for scrutiny on the clarity of the copywriting and accuracy of the examples. Also open to suggestions if you have different ideas for including commented-out values in config files (if they should be there at all).

[scottisloud]

Added defaults and improved some of the language. Resolved an issue where one of the environment variable names was incorrect.