[Docs] Document App Service slot deployment behavior and environment variables

[vhvb1989]

Context

PR #7530 (fixing #7365) introduces AZD_DEPLOY_{SERVICE}_IGNORE_SLOTS and also documents the previously-undocumented AZD_DEPLOY_{SERVICE}_SLOT_NAME in the CLI reference (cli/azd/docs/environment-variables.md).

The official docs on learn.microsoft.com should be updated to cover:

Requested Documentation

1. Slot deployment behavior — Document the automatic slot selection rules:

   • First deployment: deploys to main app + all slots
   • Subsequent with no slots: main app only
   • Subsequent with 1 slot: auto-selects that slot
   • Subsequent with 2+ slots: prompts user or uses AZD_DEPLOY_{SERVICE}_SLOT_NAME

2. AZD_DEPLOY_{SERVICE}_SLOT_NAME — Existing env var for selecting a specific slot when multiple exist. {SERVICE} is the uppercase service name with hyphens replaced by underscores.

3. AZD_DEPLOY_{SERVICE}_IGNORE_SLOTS — New env var that bypasses slot detection and deploys directly to the main app. Useful for CI pipelines that need to deploy to the main app while slots exist.

Related

• Fixes context from [Issue] azd deploy automatically deploys to a deployment slot instead of the main app when AZD_DEPLOY_{service}_SLOT_NAME is not set #7365
• Implementation in feat: Add AZD_DEPLOY_{SERVICE}_IGNORE_SLOTS env var to bypass slot deployment #7530