Documentation on 409 status codes and rate limiting

[kepol]

Is your feature request related to a problem? Please describe.

Sometimes we see a 409 status code when provisioning sites. This can happen when certain provisioning steps are rate-limited by upstream services.

Describe the solution you'd like

Unfortunately some of these limits are outside Quant's control, but:

1. While it was not possible to increase the limits for these upstream services last time we looked into this, we should review these limitations from time-to-time to see if we can bump limits in the future.

2. For now, we should add documentation for this scenario so people know why this is happening and how to possibly work around it.

Describe alternatives you've considered

If you are checking the response code during provisioning before moving onto the next step, one workaround during provisioning would be to poll the Quant preview domain programmatically to ensure an expected status code (e.g. 200, 401) before proceeding to the next stage of the provisioning process.

Additional context

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