Update digitalocean docs to include alternate droplets

Make note of smaller/cheaper droplets for DigitalOcean and document it.

Added documentation to indicate support of a smaller/cheaper droplet size for DigitalOcean hosted VPN.

Description

Added comments/indication of droplet pricing & regional availability to config.cfg and the docs/cloud-do.md files. In the documentation/setup file, I added a new subsection "Select a Droplet (optional)" consistent with other documentation writings. It explains the difference, justification, and that it's an optional change with links to DigitalOcean references (e.g. FAQs & api details).

Motivation and Context

Last year DigitalOcean expanded their offerings to add a smaller footprint droplet, s-1vcpu-512mb-10gb, that's now $4/mo. with the previous smallest droplet (and algo default), s-1vcpu-1gb, increasing to from $5 to $6/mo. The extra size is really not necessary to run the VPN, but I didn't change the default size because that droplet size isn't available in all regions. Thus, it's just indicated in the documentation

How Has This Been Tested?

Created numerous algo vpn instances via DigitalOcean droplets at sizes s-1vcpu-512mb-10gb and s-1vcpu-1gb successfully.

Types of changes

[] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Updated documentation.

Checklist:

[x] I have read the CONTRIBUTING document.
[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[] I have added tests to cover my changes.
[x] All new and existing tests passed.

trailofbits / algo