docs: improve cross-platform contributor onboarding

[kcelestinomaria]

Summary

• Keep dependency patch and diff files in LF format across platforms.
• Document the Windows build prerequisites and a PowerShell getdeps.py build command.
• Replace vague contributor validation guidance with concrete build, test, and formatting commands.
• Add PowerShell commands for running the Windows echo sample.

Motivation

The repository supports Windows, but the contributor-facing documentation did
not explain the required C++ toolchain or provide Windows-native examples. In
addition, dependency patches can be checked out with Windows line endings,
which can cause git apply to reject an otherwise valid patch.

These changes improve first-time contributor onboarding without changing
mvfst runtime behavior or public APIs.

Validation

• Verified the contribution branch is based on the latest upstream main.
• Ran git diff --check against upstream/main.
• Verified dependency patch files resolve to text eol=lf through Git attributes.
• Reviewed the documented commands against the repository's supported
  getdeps.py build and test workflow.

No C++ source or runtime behavior was changed.