feat: disable signup

[onehassan]

Checklist

• [X] No breaking changes
• [X] Tests pass
• [X] New features have new tests
• [x] Documentation is updated

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 7da8b70261a840b86a65505f86a8e4e67603e147

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------- | ----- | | hasura-auth | Minor |

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[dbarrosop]

can you update docs/environment_variables?

[onehassan]

I've added AUTH_DISABLE_NEW_USERS to docs/environment-variables.md. Should I add a small description of how to use it to docs/configuration.md?