docs(pocketid-provider): add container/Kubernetes installation guide

[gavinmcfall]

Summary

Add comprehensive documentation for running the PocketID provider plugin in containerized environments (Docker, Kubernetes).

Changes

• Added standard installation steps
• Added container/Kubernetes specific installation guide
• Documented all environment variables with descriptions
• Added Kubernetes Secret example
• Added Docker Compose example
• Documented user linking behavior (auto-link vs manual link)
• Noted the composer dependency requirement for containerized deployments

Why

The existing README was minimal and didn't explain how to configure the plugin in containerized environments where setting environment variables is the primary configuration method. Users deploying Pelican on Kubernetes or Docker need to know:

1. Which environment variables to set
2. That the socialiteproviders/pocketid composer package is required
3. How to configure the callback URL in PocketID
4. How auto-create and auto-link user features work

[gavinmcfall]

Note: The CI failure is a pre-existing issue with the repo's composer.lock being out of sync with dependencies (firebase/php-jwt version mismatch). This PR only modifies the README.md file and does not affect any code or dependencies.

[Boy132]

Thanks, but the install issues when using dockerized panel will be resolved eventually.
There should be no differences for plugin installations due to the panel installation method.

[gavinmcfall]

Is there a timeline for that? people may very well appreciate the workaround.
Happy to adjust the PR to show this is a temporary workaround