Document new OIDC variables after gristlabs/grist-core#883

[fflorent]

New variables have been introduced in this PR:

• GRIST_OIDC_IDP_ENABLED_PROTECTIONS
• GRIST_OIDC_IDP_ACR_VALUES
• GRIST_OIDC_IDP_EXTRA_CLIENT_METADATA

This PR document them.

Fixes #380

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