docs: simplify provider docs and add comprehensive cloud example

Rewrite the provider-cloud.tf example to show a complete Grafana Cloud
setup using only 2 provider blocks (cloud alias + default). Wire all
service URLs from the stack resource outputs (including the new
cloud_provider_url, connections_api_url, and sm_url attributes).

Simplify templates/index.md.tmpl by removing 6 sections (Synthetic
Monitoring installation, OnCall, Frontend O11y, Cloud Provider,
Connections, Fleet Management) that duplicated individual resource docs.

Update oncall_url and oncall_access_token descriptions in both framework
and legacy providers to clarify they are only needed for OnCall OSS or
dedicated tokens, not for Grafana Cloud usage.

Also update replace_references.go with new cross-references generated
from the updated example.

Author: Duologic