[WXP] Document how to change the availability status of context menu items

[samantharamon]

Note to the reviewer: Unified manifest updates will be made in a separate PR since this PR may be published before WXP public preview.

[learn-build-service-prod]

Learn Build status updates of commit 7d699eb:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/design/disable-add-in-commands.md	💡Suggestion	View	Details
docs/design/add-in-commands.md	✅Succeeded	View
docs/design/contextual-tabs.md	✅Succeeded	View
docs/design/interface-elements.md	✅Succeeded	View
docs/develop/configure-your-add-in-to-use-a-shared-runtime.md	✅Succeeded	View
docs/toc.yml	✅Succeeded	View

docs/design/disable-add-in-commands.md

• Line 31, Column 24: [Suggestion: learn-site-link-broken - See documentation] Link '/javascript/api/requirement-sets/common/context-menu-api-requirement-sets' points to a learn site page that doesn't exist. Check the path or URL and update the link.
• Line 140, Column 13: [Suggestion: learn-site-link-broken - See documentation] Link '/javascript/api/office/office.contextmenuupdaterdata' points to a learn site page that doesn't exist. Check the path or URL and update the link.
• Line 143, Column 101: [Suggestion: learn-site-link-broken - See documentation] Link '/javascript/api/office/office.contextmenu#office-office-contextmenu-requestupdate-member(1)' points to a learn site page that doesn't exist. Check the path or URL and update the link.

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