Document supported environment variables

[EMaher]

Support for the following environment variables should be added to documentation:

Found in code:
AZURE_SUBSCRIPTION_ID — fallback for --subscription-id on extract and publish
AZURE_API_VERSION — overrides the APIM REST API version on extract and publish
COMMIT_ID — enables incremental publish unless overridden by --commit-id
AZURE_CLIENT_ID — auth via DefaultAzureCredential
AZURE_CLIENT_SECRET — auth via DefaultAzureCredential
AZURE_TENANT_ID — auth via DefaultAzureCredential

Also documented in the CLI contract/README as telemetry-related:
OTEL_EXPORTER_OTLP_ENDPOINT
APPLICATIONINSIGHTS_CONNECTION_STRING