docs: update AddASetting guide

[Darius B. (journaltraces)]

Updates doc/cascadia/AddASetting.md to match the current Terminal Settings Model implementation.

The guide previously described adding profile and global settings using older patterns, including manual setting declarations in Profile.h, manual copy/JSON handling in Profile.cpp, and standalone WINRT_PROPERTY declarations in TerminalSettings.h

This update documents the current settings-list and IDL-based architecture, including:

• MTSM_PROFILE_SETTINGS
• MTSM_GLOBAL_ONLY_SETTINGS
• MTSM_WINDOW_SETTINGS
• INHERITABLE_SETTING
• INHERITABLE_PROFILE_SETTING
• terminal-facing settings through IControlSettings / ICoreSettings
• ControlProperties.h and TerminalSettings packaging
• current ProfileViewModel projection patterns

Fixes #20550

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[Darius B. (journaltraces)]

@microsoft-github-policy-service agree

[Leonard Hecker (lhecker)]

I did not cross-read against the old version, but the new version is overall good.

[Darius B. (journaltraces)]

I’ve pushed a commit addressing all four comments.