[3.0] Contributor experience improvements

[Perksey]

• Adds a basic for-contributors getting started README for building the repo, detailing what you need to get started. Closes Create building-from-source.md #2403 which also does the same.
• Adds a "best effort" attempt to resolve native binaries when used as a ProjectReference.
• Adds the ability to disable building for Android/iOS by adding properties to Directory.Build.props which are then used when setting TargetFrameworks

[github-actions]

Some of the native library builds modified in this PR were not referenced in the PR description. Please ensure that the PR description contains /build-native SDL. These libraries won't be rebuilt without this being specified. If you believe this is in error, then please write a comment explaining why and ignore this suggestion. This comment will be automatically updated if rectified.

[Perksey]

The PR check is triggering because I've modified the native project to not include an Android build if disabled. This does not warrant a rebuild.

[Perksey]

@curin Ready for review.

[github-actions]

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[Perksey]

@joskuijpers hopefully this helps? PTAL

[joskuijpers]

Good changes! Playground now runs without SDL DLL issues, and the contributor document was very useful too.