Document: Track deployments of external deployment tool

What does this MR do and why?

This MR documents the feature added for Add external environments and deployments to Gi... (#22513 - closed).

This effectively addresses the following questions/confusions:

• https://gitlab.slack.com/archives/C01NKFJ4HUP/p1668659949291879?thread_ts=1668451436.789629&cid=C01NKFJ4HUP
• https://gitlab.slack.com/archives/CBZGR90A2/p1668461447880239

Delivery Team uses API to add and update deployments and would like more guidance on how to manage the lifecycle for deployments (which states should be used when)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Related #382729 (closed) #276248

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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