docs: clarify JAVA_HOME and initial build expectations on Windows

[Esmelealem]

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[gemini-code-assist]

Summary of Changes

Hello @Esmelealem, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the developer onboarding experience for Windows users by providing clearer and more detailed instructions for setting up the Java Development Kit (JDK) and managing initial build expectations. The changes address common pitfalls related to JAVA_HOME configuration and inform users about the typical duration for the first Gradle build, aiming to reduce friction for new contributors.

Highlights

• Java Setup on Windows: Added specific instructions for Windows users regarding the JAVA_HOME environment variable, clarifying it should point to the JDK installation directory, not the bin directory, and advising to prioritize Java 11 in the PATH if multiple JDKs are present.
• Initial Build Expectations: Included a note informing Windows users that the initial Gradle build may take 10-15 minutes due to dependency downloads.

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[shivanshjais22]

Hi @apache-beam team 👋
I would like to work on this issue.

I am using Windows and Java, and I am currently setting up Apache Beam locally.
I will try to improve the documentation related to JAVA_HOME and Windows build steps.

Please let me know if I can take this issue and if there are any guidelines I should follow.
Thanks!

[Esmelealem]

Hello Beam maintainers 👋

Just a gentle follow-up on this PR.
This change addresses JAVA_HOME and initial build expectations for Window users.
Happy to make any changes, add tests, or adjust the approach if needed.

Thanks for your time and for maintaining Beam!

[Esmelealem]

assign to next reviewer

[Esmelealem]

CI is green. PTAL when you have a chance — thank you!