docs: Updates for Teams CLI introduction and navigation cleanup

[heyitsaamir]

Summary

Documentation updates to introduce the Teams CLI, restructure navigation, and clean up stale references across the SDK docs.

• New Get Started section with two quickstarts: Register your app (CLI/infra, language-agnostic) and Build your first bot (SDK, tabbed TypeScript/C#/Python)
• Sidebar restructured to explicit Diátaxis-style layout: Get Started → Teams Platform → Language Guides → Developer Tools
• Azure Configuration page replaces the old manual-configuration and agents-toolkit pages under Teams Platform
• Copy for LLM button added to every doc page via swizzled DocItem/Layout; respects llms: ignore frontmatter
• Agent Skills page added to CLI Getting Started documenting the teams-dev skill for AI coding assistants
• ATK references updated to Teams CLI throughout; Running in Teams now shows both paths (Teams CLI vs ATK) side by side
• Broken links fixed across templates and main docs (deleted pages, hardcoded paths, stale ATK links)
• Navigation improved with "What's in this section" links added to AI, Observability, Migrations, BotBuilder, and DevTools sections
• Language-specific Quickstart renamed to "Local Setup" to avoid naming conflict with the new top-level quickstart

Stacked on top of #2744 (add-teams-cli).

[umangsehgal]

Superseded by #2786, which adapts this work to the published Teams CLI v2.1 Preview docs site. Closing in favor of the new PR.