[docs] guidance before selecting the tutorial scenarios

[Nobuto Murata]

We have 4 tutorial scenarios in the official doc:
https://microstack.run/docs

- Single-node quickstart
- Single-node guided
- Multi-node (local)
- Multi-node with MAAS

It might be a good idea to have an overview of those 4 scenarios so it can help visitors to select a suitable scenario for them. I know the following the table is incomplete but I mean something like this.

| Scenario | Single-node quickstart | Single-node guided | Multi-node (local) | Multi-node with MAAS |
| --- | --- | --- | --- | --- |
| \# of nodes | 1 | 1 | 3+ | 4+ |
| Large scale | \- | \- | \- | ✓ |
| Redundancy | \- | \- | ✓ | ✓ |
| VM access over network | \- | ✓ | ✓ | ✓ |
| Minimum CPU cores per node | 4 | 4 | 4 | 4 |
| Minimum memory per node | 16GB | 16GB | 32GB | 32GB |
| \# of drives per node | 1 | 1 | 2 | 2 |
| \# of NIC ports per node | 1 | 2 | 2 | 2 |