Pull ESC REST APIs into Pulumi ESC Section

[arunkumar611]

[TBD] Let's wait till we make a decision on this.

Moving these APIs from https://www.pulumi.com/docs/pulumi-cloud/cloud-rest-api/#environments

to

https://www.pulumi.com/docs/esc/ add a new page: esc-rest-api

[toriancrane]

So far from what I can see on that thread, I'm a fan of the idea of moving the REST API to top level, and then in the ESC section just having tutorial content for how to use it for ESC.