docs: HTML Widgets in-depth guide

[corinagum]

Do not merge: putting on hold per request of HTML Widgets PM.

Summary

Adds documentation for the HTML Widgets feature (in-depth guide) covering all three SDKs (TypeScript, Python, .NET). HTML Widgets let an app render rich, interactive HTML inside a Teams message via an MCP Apps (ext-apps) surface.

Pages added

Under In-Depth Guides > HTML Widgets:

• Overview (README.mdx) - what HTML Widgets are, how they travel inside an extendedmarkdown message, the payload shape, and a "What the SDK provides" helper index.
• Sending Widgets - building a widget message/markdown with the build helpers.
• Handling Tool Calls - the typed htmlwidget/calltool handler and typed response shape.
• Security Policy - securityPolicy, the validate/inject helpers, and their limits.
• Updating Model Context - parsing a widget's ui/update-model-context request off an incoming message.

Notes

• Every page carries a Preview admonition (the feature and its API surface are still in preview).
• The Security Policy page includes a "Not a security boundary" note clarifying the helpers are development-time aids, not a replacement for the client's runtime enforcement or your own HTML review.

[Copilot]

Pull request overview

Adds a new HTML Widgets in-depth documentation section to teams.md, covering the end-to-end flow (sending widgets, handling tool calls, security policy helpers, and model-context updates) with per-language include snippets (TypeScript/Python/.NET).

Changes:

• Adds the HTML Widgets overview page with payload shape + navigation to sub-guides.
• Adds four sub-guides (sending widgets, tool calls, security policy, updating model context).
• Adds language-specific include snippets for each guide section, plus the new doc category definition.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
teams.md/src/pages/templates/in-depth-guides/html-widgets/README.mdx	Overview page: concept, wire format, payload fields, and SDK helper index.
teams.md/src/pages/templates/in-depth-guides/html-widgets/sending-widgets.mdx	Guide page for constructing/sending widget messages and markdown.
teams.md/src/pages/templates/in-depth-guides/html-widgets/handling-tool-calls.mdx	Guide page for receiving htmlwidget/calltool invokes and returning typed results.
teams.md/src/pages/templates/in-depth-guides/html-widgets/security-policy.mdx	Guide page for securityPolicy and the validate/inject helpers.
teams.md/src/pages/templates/in-depth-guides/html-widgets/updating-model-context.mdx	Guide page for parsing ui/update-model-context requests from message activities.
teams.md/src/pages/templates/in-depth-guides/html-widgets/category.json	Adds the “HTML Widgets” category metadata (sidebar position/label).
teams.md/src/components/include/in-depth-guides/html-widgets/sending-widgets/typescript.incl.md	TS snippets for building widget messages/markdown.
teams.md/src/components/include/in-depth-guides/html-widgets/sending-widgets/python.incl.md	Python snippets for building widget messages/markdown.
teams.md/src/components/include/in-depth-guides/html-widgets/sending-widgets/csharp.incl.md	C# snippets for building widget messages/markdown.
teams.md/src/components/include/in-depth-guides/html-widgets/handling-tool-calls/typescript.incl.md	TS snippets for handling tool calls + response shape.
teams.md/src/components/include/in-depth-guides/html-widgets/handling-tool-calls/python.incl.md	Python snippets for handling tool calls + response shape.
teams.md/src/components/include/in-depth-guides/html-widgets/handling-tool-calls/csharp.incl.md	C# snippets for handling tool calls + response shape.
teams.md/src/components/include/in-depth-guides/html-widgets/security-policy/typescript.incl.md	TS snippets for validating policy + injecting protocol script.
teams.md/src/components/include/in-depth-guides/html-widgets/security-policy/python.incl.md	Python snippets for validating policy + injecting protocol script.
teams.md/src/components/include/in-depth-guides/html-widgets/security-policy/csharp.incl.md	C# snippets for validating policy + injecting protocol script.
teams.md/src/components/include/in-depth-guides/html-widgets/updating-model-context/typescript.incl.md	TS snippet for parsing ui/update-model-context.
teams.md/src/components/include/in-depth-guides/html-widgets/updating-model-context/python.incl.md	Python snippet for parsing ui/update-model-context.
teams.md/src/components/include/in-depth-guides/html-widgets/updating-model-context/csharp.incl.md	C# snippet for parsing ui/update-model-context.

[nickwalkmsft]

Who is the feature PM for this? Widgets in general aren't on the docs backlog.

Sorry, forgot my general sentiment - this is great! Most of these comments are questions.