Add user documentation for Compiler Extensions

### 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: https://github.com/getsentry/sentry-dotnet/pull/4840

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

- Functionality and Intent of the Source Generator
- document how to disable it in case of issues
- see https://github.com/getsentry/sentry-dotnet/blob/50186bec21b03504d0cca3f9f5a52b1c80f81675/src/Sentry.Compiler.Extensions/BuildPropertySourceGenerator.cs#L34
- Functionality and Intent of the Analyzer
- document how to suppress this Diagnostic (ID: `SENTRY1001`)
- document how to suppress all Sentry Diagnostics (Category: `Sentry`)
- see also https://github.com/getsentry/sentry-dotnet/pull/4840#discussion_r2860110429