Improve deployment how-to landing page

`docs/how-to/deploy/index.md`

Add some brief context about the different ways of deploying MongoDB:
* VM vs K8s
* Replica set vs sharded
* Juju vs Terraform
* Other clouds

Start by reading docs/how-to/deploy/index.md and the deployment how-to material it introduces. Identify the existing deployment options covered in the repository, then add brief context comparing VM and Kubernetes, replica sets and sharding, Juju and Terraform, and other clouds. Done means the landing page helps readers choose among these approaches without changing the deployment instructions.

Written by the indexing model from the issue text.