docs: Add SingleStore to the list of databases that can be enrolled with teleport

[pmishchenko-ua]

SingleStore uses mysql protocol to talk to the dataabse server, therefore MySQL integration works with SingleStore with a few changes:

• SingleStore-specific database configuration
• Export of teleport CA is needed in addtion to server.cas, server.crt, server.key.

When I was following mysql docs, it was unclear what to do with the instructions on user's role

allow:
  impersonate:
    users: ["Db"]
    roles: ["Db"]

so I've expanded the corresponding part of the docs.

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