Add documentation for ios.uiscene build hint

[shai-almog]

Motivation

• Add documentation for a new iOS build hint ios.uiscene which enables experimental UIScene lifecycle support to help apps prepare for upcoming iOS lifecycle changes.

Description

• Documented the new ios.uiscene build hint in the advanced topics guide as a true/false flag with a default of false and described its purpose and rationale.

Testing

• No automated tests were run for this documentation-only change.

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Codex Task

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF
• AsciiDoc linter report
• Vale report
• Unused image report

Developer Guide quality checks:

• AsciiDoc linter: No issues found (report)
• Vale: 18824 alert(s) (2768 errors, 6384 warnings, 9672 suggestions) (exit code 1) (report)
• Image references: No unused images detected (report)