Add blog post: App Service Build 2026 Updates

[btardif]

Adds the App Service Build 2026 updates rollup blog post.

What's in this post

• Easy AI — new AI (preview) blade, Built-in MCP on App Service, and the Agents tab for AI telemetry.
• Isolated v4 on ASEv3 — GA announcement and what it unlocks for ASE workloads.
• Managed Instance on App Service — recent preview improvements and the path to MI on ASE via Iv4.
• Platform Release Channel — Latest / Standard / Extended channels for controlling runtime patch rollout on Linux.
• AZ CLI improvements — enriched deployment errors, revamped list-runtimes, and CLI backlog burn-down.
• Platform Improvements — Python deployment pipeline (~30% faster), FastAPI auto-detection, Deferred Kudu Recycle, SSH helper aliases, and Site Status.

Each section links out to the dedicated blog posts for the deep dive.

Notes for reviewers

• Front matter follows the existing convention used by recent posts in _posts/.
• All section anchors in the intro have been wired to their corresponding H2s.
• A couple of TODOs from the source doc were intentionally left out (Available Stacks API blurb pending content); easy to add in a follow-up.

[github-actions]

Preview link: https://antares-blog-staging-pr-551.azurewebsites.net

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