Add documentation for digitalocean runner configuration according to the new DO plans

After using gitlab runner with digital ocean servers I was surprised that I was charged twice comparing to the amount I expected.

I contacted Digital ocean support and this is what they replied:

Thanks for contacting us. Sorry for the confusion here. It looks like whatever tool you are using to create those droplets is not using the correct slug names. The new droplet plans have different named slugs: https://developers.digitalocean.com/documentation/changelog/api-v2/new-size-slugs-for-droplet-plan-changes/

If you can change those names in your build then it should resolve that. If you are using a third party tool we would recommend reaching out to them to update their software to use the correct new droplet slug names.

Before I used the following MachineOptions:

MachineOptions = [
  "digitalocean-size=s-2gb",
  ...
]

After I upgraded the configuration to the following:

MachineOptions = [
  "digitalocean-size=s-2vcpu-4gb",
  ...
]

gitlab runner started to create standard droplets instead of high CPU ones to run jobs

The problem is that I could not find any information about how to configure gitlab runner on Digital ocean except a bit on this page https://docs.gitlab.com/runner/configuration/advanced-configuration.html

There is a documentation how to configure autoscaling gitlab runner on AWS

What do you think about adding a page about autoscaling gitlab runner on Digital ocean, so for the users who configure it for the first time would be less surprises?

Thank you!