Add Global Secure Access Client public download links to docs

[FaserF]

This pull request will add the public download links for the Global Secure Access Client and all its operating system versions.
This is needed for this PR to get mergered: microsoft/winget-pkgs#172809

[prmerger-automator]

@FaserF : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 1232818:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/global-secure-access/how-to-install-windows-client.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[ttorble]

@HULKsmashGithub

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[HULKsmashGithub]

@ttorble and @FaserF , I'm getting input from the product manager on these edits. I'll post again when I've heard back.