Add user documentation for Compiler Extensions

[Flash0ver]

Description

We currently have two compiler extensions:

• a Source Generator: see https://github.com/getsentry/sentry-dotnet/blob/main/src/Sentry.Compiler.Extensions/BuildPropertySourceGenerator.cs
• a Diagnostic Analyzer: feat(metrics): Trace-connected Metrics (Analyzers) #4840

Let's add some documentation with a brief description, example, and when/how to disable / suppress these.

[linear]

NET-598