managed instances: overhaul / clarify / update customer-facing docs

Created by: slimsag

This PR overhauls the managed instances doc to be more clear in some areas and specify other areas in more detail.

Notable changes include:

• It is now clear we support IP-based VPN restrictions and this is not a special request.
• Technical security has been clarified.
• Broke down the services by general category so it is easier to skim-read over, if desired.
• Clarified that there are on-call engineers monitoring the health of managed instances.
• Declared that we will regularly reassess resource utilization and costs.
• Described some more of the initial benefits you get with this service, such as resource estimation, managed DNS/HTTPS, software/kernel configuration, etc.
• Declared that we only support Docker Compose managed instances today, we may revisit this in the future if there is a need but Docker Compose seems to scale well enough for most medium-large enterprise customers so in the meantime we should consider such a situation as a special request and/or suggest doing a self-hosted instance in such cases.
• Clarified what the difference in pricing actually is between Enterprise vs. managed enterprise (previously just "contact us".)