Update marketplace (MP) docs to clarify sign up steps

[aashishkohli]

Areas to address:

1. Show public offer sign up steps for AWS and GCP, Azure coming soon
2. Show private offer acceptance steps for AWS, GCP (azure coming soon)
3. For #1 and #2, clarify that customers need to sign up on CH Cloud after subscribing on MP
4. Show steps to switch from direct to MP billing, see below: For customers that are already subscribed to CH Cloud but then want the billing to happen through MP for the same org, we have this section: https://clickhouse.com/docs/knowledgebase/marketplace#i-am-an-existing-clickhouse-cloud-user-and-i-want-my-existing-services-to-be-billed-via-marketplace

However, is it not clear from the first section whether a user needs to take additional steps to connect an existing CH Cloud org to a MP subscription. We need to clarify this. I will provide updated sections.

[aashishkohli]

https://clickhouse-inc.slack.com/archives/C04ASGUS7TJ/p1708714635850119

[aashishkohli]

@justindeguzman - I will provide a google doc with updated marketplace docs. We should just replace the current docs page with the updated one.