update deployment guide with a few more details

[hrodmn]

Here are a few suggested changes to the deployment guide based on my experience with it yesterday. This is a really great tool, I'm excited to replace my cobbled together stac-fastapi-pgstac + titiler-pgstac deployment with this!

The main change I needed to get things running was to add the CDK_EOAPI_STAGE env var to deployment commands because it determines the name of the stack and it needs to be consistent across the steps.

[vincentsarago]

🥳 thanks @hrodmn , really appreciated