maint: add macOS & Windows system files to .gitignore

[vladdoster]

• ignore the macOS .DS_Store & Windows Thumbs.db files
• remove .DS_Store

See examples: MicrosoftDocs/azure-docs and MicrosoftDocs/semantic-kernel-docs

[learn-build-service-prod]

Learn Build status updates of commit f377b3e:

✅ Validation status: passed

File	Status	Preview URL	Details
.DS_Store	✅Succeeded	n/a (file deleted or renamed)
.gitignore	✅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

[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.
Unapproved file extension(s)	This PR contains file types that are not supported in the repo. The only supported file types are jpg, gif, png, json, svg and md.
Repository root file	This pull request contains a file submitted to the root of the repository.

[learn-build-service-prod]

Learn Build status updates of commit 1eb8f50:

✅ Validation status: passed

File	Status	Preview URL	Details
.DS_Store	✅Succeeded	n/a (file deleted or renamed)
.gitignore	✅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

[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.
Unapproved file extension(s)	This PR contains file types that are not supported in the repo. The only supported file types are jpg, gif, png, json, svg and md.
Repository root file	This pull request contains a file submitted to the root of the repository.