[Doc] Backend: document the Edge connection architecture (Backend.Edge.App)

[arindahills]

The backend Architecture page was empty. This documents how an OpenEMS Edge connects to the Backend:

• the Edge-facing endpoint was split out of the central Backend into its own scalable application, Backend.Edge.App;
• an Edge always connects (via its unchanged Controller.Api.Backend) to a Backend.Edge.App, which relays to the central Edge.Manager;
• deployment options: one or more Backend.Edge.App on separate servers, or a single one on the central server via localhost.

This follows up on the discussion in #3795, where this setup was noted as not yet documented. Happy to adjust the wording or add a diagram image if preferred.

[sfeilmeier]

Thank you, this is very helpful! 👍

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #3818      +/-   ##
=============================================
- Coverage      60.48%   60.44%   -0.04%     
  Complexity       105      105              
=============================================
  Files           3456     3456              
  Lines         151072   151072              
  Branches       11528    11528              
=============================================
- Hits           91363    91300      -63     
- Misses         56265    56336      +71     
+ Partials        3444     3436       -8     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.