feat: add Rauthy OIDC Provider configuration instructions

[pyrox0]

Description

Adds configuration instructions for using Rauthy with Grimmory. I encountered a few issues when configuring it for my setup, so I'd figure I'll document these so no one runs into the same errors I did.

Few more things before this is ready to merge:

• Should the attribute instructions be embedded in grimmory's docs? I'd rather link upstream since they may change in the future.
• How do I update the screenshots? I'd rather not set up an entire grimmory testing instance to get a few grimmory screenshots, since my prod instance is customized theme-wise and using the default theme seems like a better idea.

Linked Issue

N/A

Changes

Adds Rauthy configuration to the OIDC Providers section

Manual Testing Steps

1. Open the docs site
2. Look in the sidebar and see the new page
3. Optionally follow the steps to configure the provider.

Additional Context (Optional)

N/A

AI Disclosure

None

Checklist

• This PR links and implements an accepted issue.
• This PR is a single focused change.
• I ran the relevant validation for this change, such as npm run build and/or npm run typecheck.
• I have added screenshots if there were any UI changes.
• I have disclosed any AI usage as per the organization AI Policy above.
• I understand all of my submitted changes.

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