docs: fold the permission casing discussion into the FC0006 page

[Arthurvdv]

Summary

Rewrites the FC0006 (Permission values should be lowercase) page based on ALCops/Analyzers#560 and the material in ALCops/Analyzers#383 (including the original Viva Engage thread).

• The Why now explains the two-condition indirect-permission handshake and why casing plays no part in either check, with Stefan Maroń's "don't actually elevate anything" quote inline.
• New section contrasting the Learn values table (uppercase = direct) with the runtime results table further down the same page, which only distinguishes set / not set. Also notes the lowercase rm in Learn's own example and the historical casing mix in the base app.
• New section on Permissions declared on pages, quoting Learn on indirect read and pages.
• New section on verifying the behavior: Docker/developer-license caveat, cloud sandbox, TestPermissions = Restrictive.
• When the diagnostic is reported derived from the test fixtures; Code fix mentions the AC0031 fix emitting lowercase versus AZ AL Dev Tools emitting uppercase.
• See also links Hougaard and discussion #383.

Verification

• hugo --minify builds cleanly.
• [params] checked against DiagnosticDescriptors.cs, the code fix provider, and the IsObsolete() guard.
• All Learn anchors used in attributions verified on the live page.

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