website/docs: add docs for source switch expression policy (backport of #16878)

[authentik-automation]

⚠️ This cherry-pick has conflicts that require manual resolution.

Cherry-pick of #16878 to version-2025.6 branch.

Original PR: #16878
Original Author: @tanberry
Cherry-picked commit: dbf85d0

Please resolve the conflicts in this PR before merging.

[netlify]

❌ Deploy Preview for authentik-integrations failed. Why did it fail? →

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🔨 Latest commit	e5c0673
🔍 Latest deploy log	https://app.netlify.com/projects/authentik-integrations/deploys/68d3d83b73697e00080146d5

[netlify]

❌ Deploy Preview for authentik-docs failed. Why did it fail? →

Name	Link
🔨 Latest commit	e5c0673
🔍 Latest deploy log	https://app.netlify.com/projects/authentik-docs/deploys/68d3d83bb2e0ec0008bb979a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (912284b) to head (e5c0673).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                Coverage Diff                 @@
##           version-2025.6   #16974      +/-   ##
==================================================
- Coverage           92.98%   92.94%   -0.05%     
==================================================
  Files                 817      817              
  Lines               42295    42295              
==================================================
- Hits                39330    39309      -21     
- Misses               2965     2986      +21     

Flag	Coverage Δ
e2e	47.53% <ø> (-0.05%)	⬇️
integration	24.37% <ø> (+<0.01%)	⬆️
unit	90.84% <ø> (+<0.01%)	⬆️

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[tanberry]

I'll do it manually