docs(sidenav): update responsive example to use current APIs

[EduardF1]

Summary

Updates the responsive sidenav documentation example to use the current CDK APIs instead of deprecated MediaMatcher/MediaQueryList event listener methods.

Root Cause

The example was using MediaMatcher with addEventListener/removeEventListener, which requires manual lifecycle management via OnDestroy. An Angular team member noted the preferred approach is to use the CDK's BreakpointObserver.observe() directly.

Changes

• Replaced MediaMatcher with BreakpointObserver.observe()
• Used takeUntilDestroyed() for automatic subscription cleanup
• Removed OnDestroy implementation and manual event listener management
• Removed private _mobileQuery and _mobileQueryListener fields

Result

The component is now simpler, idiomatic Angular, and no longer uses the raw MediaQueryList API.

Fixes#29266

[google-cla]

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[EduardF1]

Update: I have already signed the Google CLA with fischerszavarduard@gmail.com, and I have now rewritten this PR commit so the author/committer email matches that signed address. The cla/google check is still failing. Could a maintainer please re-run or investigate?

[EduardF1]

Just following up on this PR. The checks look good (apart from the CLA which I believe needs separate handling). Let me know if I need to make any changes!

[EduardF1]

Hi! Just checking in on this PR. The CI is green. Is there anything else needed from my side to help get this reviewed and merged?

[EduardF1]

@googlebot I signed it!