[docs] Add descriptions for azd env configuration variables

[abhirockzz]

The azd env set variables used during deployment are not documented with descriptions anywhere in the repo. They appear in docs/e2e-guide.md, hooks/preprovision.sh, and infra/main.parameters.json but without explanations — users who skip the interactive preprovision prompts have no reference for what each variable controls.

Request

Add a reference table (e.g. in docs/e2e-guide.md or README.md) documenting each variable.

the table below is just a suggestion

Variable	Purpose	Valid Values
OMNIVEC_METADATA_STORE	CosmosDB throughput mode for internal metadata (sources, destinations, pipelines, jobs, tokens)	cosmosdb-serverless (default), cosmosdb-provisioned
OMNIVEC_ENABLE_BLOB_SOURCE	Whether to provision Storage Account, Service Bus, and Event Grid for Azure Blob ingestion	true (default), false
OMNIVEC_SYSTEM_NODE_VM_SIZE	AKS system node pool VM SKU	Any valid VM SKU, e.g. Standard_D4ds_v5
OMNIVEC_SYSTEM_NODE_COUNT	Number of nodes in the AKS system node pool	Integer, e.g. 2
OMNIVEC_GPU_NODE_VM_SIZE	AKS GPU node pool VM SKU	Any valid GPU VM SKU, e.g. Standard_NC6s_v3
OMNIVEC_GPU_NODE_COUNT	Number of nodes in the AKS GPU node pool (0 = no GPU pool created)	Integer, e.g. 0
OMNIVEC_BUILD	Whether to build images from source or import pre-built images from the shared registry	true (build from source), false (default — import from shared registry)
OMNIVEC_BUILD_MODE	How images are built (only applies when OMNIVEC_BUILD=true)	acr (default — remote build via ACR Tasks), docker (local Docker build + push)