BYOC Azure self-onboarding flow change

[luisliuchao]

Why

Since we've enabled the Azure self-onboarding flow for BYOC, we need to update the corresponding documentation as well

What

1. Remove Azure preview doc page in https://clickhouse.com/docs/cloud/reference/byoc/onboarding/azure-private-preview
2. Add screenshot of gcp custom network setup in https://clickhouse.com/docs/cloud/reference/byoc/onboarding/customization-gcp
3. Remove the onboarding url /byocOnboarding if exists