ece medium and large installation instructions improved

[eedugon]

This PR fixes and improves ECE medium and large installation instructions.

Fixes:

• The description of amount of allocators needed for a large installation wasn't coherent.
• We had a bug in the procedure where we were not adding the step of removing the allocator role from the initial host, which is important.
• Large instructions also needed to remove proxy role.
• Removing proxy role on first host has the implication of needing to update the endpoint URLs.

Enhancements:

• Added a small note to understand why the drawing shows the allocator role greyed out in one of the hosts. It improves the reading flow.
• Log into Cloud UI was in the wrong order :)
• Change deployment configuration link points now to the right place.

Closes https://github.com/elastic/cloud/issues/133321

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/deploy/cloud-enterprise/deploy-large-installation.md
• deploy-manage/deploy/cloud-enterprise/deploy-medium-installation.md

[kilfoyle]

LGTM! 🐅
Looks great! Just one suggestion.

[eedugon]

Thanks for your comments @kilfoyle .

I've realized that we had more bugs in the "large" doc, and I'll need to fix them before merging.

In the "large installation", we forgot to remove the proxy role (which is also greyed-out in first host).
Also by default ECE is configured with the IP address of the first installed node as the base to create the external endpoint URLs.
If that node removes the proxy role the URLs provided by cloud UI won't work, and the user needs to change the configuration of the Endpoint URL configuration in ECE -> Settings page. I'll also add a step and section to that.

[kilfoyle]

LGTM! ⛵
Looks superb @eedugon!