Add documentation and configuration for deploying Helm chart with oauth2-proxy

[DiamondJoseph]

Explains how to add oauth2-proxy as a dependency of an umbrella Helm chart, allowing the web front end and API to be served behind a reverse proxy that redirects unauthenticated traffic to the auth provider.

According to Jake (Filik), passing the Authorization header into the application requires using the unstable alphaConfig. I'd like to find a nicer way of handling the oauth2-proxy -> tiled service name mapping.

Parking here momentarily until authentication can be revisited.

Checklist

• [ ] Add a Changelog entry
• [ ] Add the ticket number which this PR closes to the comment section

[danielballan]

Power-cycled to re-run CI against main.

[DiamondJoseph]

I'd hold fire on this one for a day or two, I'm currently in progress deploying it at DLS and I've got some modifications to make