Document supported storage backends

[RJPercival]

We have storage implementations for MySQL, Cloud Spanner and (partially) Postgres. However, we don't document whether we support deployments that use them. Some helpful things to document include:

• Which storage backends do we support?
  • Do we support Cloud Spanner even though the storage implementation is marked "experimental"?
  • We presumably don't support Postgres, since the storage implementation is incomplete.
• Which versions of storage backends do we support?
  • Do we support MySQL 8?
  • Do we support variants of MySQL, e.g. MariaDB, and if so, which ones?

[RJPercival]

We should also document what it means for us to support a deployment (e.g. SLO on support requests only for supported deployments?).

[RJPercival]

Also to consider: whether we only support specific configurations of some of these storage backends, e.g. only MySQL backed by SSD.

[mhutchinson]

These are partially documented at https://github.com/google/trillian/blob/master/docs/Feature_Implementation_Matrix.md and this would benefit from fleshing out. See #1845 for an example of uncertainty and issues around specific versions of DB.

[mhutchinson]

I'm going to tidy up the feature implementation matrix and call this done.