docs: add self-hosted MS Teams setup guide

[masami-agent]

Description

Adds a self-hosted MS Teams setup guide based on @neilkuan's approach from the feat/teams-gateway branch.

This doc covers the self-hosted deployment path using Docker Compose + Cloudflare Tunnel (or any reverse proxy), targeting individuals and small teams who want to connect MS Teams to OpenAB without enterprise Azure infrastructure.

What's included

• Azure AD app registration walkthrough
• Azure Bot resource creation
• Teams app manifest (Developer Portal + hand-rolled JSON)
• Docker Compose stack (gateway + OAB + optional Cloudflare Tunnel)
• Public HTTPS exposure options (Cloudflare Tunnel, ngrok, Tailscale Funnel, etc.)
• Environment variables reference
• Troubleshooting guide

Context

This is part of the PR #550 split discussed with @obrutjack:

1. PR refactor(gateway): split main.rs into adapter modules #550 → slim down to gateway adapter modular refactor only
2. This PR → self-hosted MS Teams setup doc
3. Future PR → enterprise MS Teams deployment (Helm chart + AKS docs)

Notes

• All sensitive values use <YOUR_xxx> placeholders
• No code changes — docs only
• Based on docs/msteams.md from feat/teams-gateway branch (credit: @neilkuan)

[github-actions]

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[masami-agent]

Superseded by #667 — self-hosted docs are now included alongside the Teams adapter code in a single PR.