README: Darling runs on Linux and monitors PostgreSQL

[erikdarlingdata]

What does this PR do?

The root README described Darling as a Windows service for SQL Server with one viewer. That was out of date. Darling/README.md says three things that the root README did not:

• The service runs on Linux too, under Docker Compose or systemd. Each release has a Linux tarball.
• Darling monitors PostgreSQL targets: self-hosted, AWS RDS, and Aurora.
• Darling has a web dashboard as well as the Windows viewer.

This PR adds these facts to the root README. It changes only the text that stated them wrong or left them out:

• Supported line: adds "PostgreSQL, including AWS RDS and Aurora (Darling)".
• Editions table: "Windows or Linux", the web dashboard, PostgreSQL in "Best for", a link to the PostgreSQL grants in "Requires", and "bundled on Windows" for the store.
• The alerts sentence under the editions table: the system tray is for Lite and Dashboard only. Darling alerts by email and webhooks.
• Darling quick start: "Windows or Linux", SQL Server and PostgreSQL servers, and the viewer reads the store from a Windows seat. Step 2 tells the reader how to add a PostgreSQL server. A new paragraph after the steps sends Linux users to the "Run on Linux" section of the operator guide.
• Edition comparison table: "Windows or Linux", a new PostgreSQL row, "bundled on Windows", and the Linux tarball.
• Tabs section: the Darling viewer shows a PostgreSQL target on its own set of tabs.

llms.txt got the same corrections in #4157.

Which component(s) does this affect?

• Lite
• Darling
• Lite Tests
• Darling Tests
• SQL collection scripts
• Documentation
• Full Dashboard (deprecated)
• CLI Installer (deprecated)

How was this tested?

• The Lite test classes that read the root README pass: CrossAppMcpToolInventoryPinTests, EntraDefaultCredentialTests, EntraDeviceCodeTests, LiteRuntimePrerequisiteDocsTests, and CrossAppGuardCiGateTests (119 tests).
• The Darling tests that read the root README check the CPU and TempDB alert rows and one field name. This PR does not change those rows, and it does not add that field name.
• The two new links point at headings that exist in Darling/README.md. docs/uat-onboarding.md already links the "Run on Linux" anchor.

Checklist

• I have read the contributing guide
• My code builds with zero warnings (dotnet build -c Debug)
• I have tested my changes against at least one SQL Server version (not applicable, because only the README changed)
• I have not introduced any hardcoded credentials or server names

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