Add IPv6 only cluster docs

[Charlio99]

Signed-off-by: Carlos Mora

Why do you need this?

• I have raised an issue to propose this change (required)

IPv4 VPS are getting expensive, therefore, now VPS are cheaper if you decide to go for the only IPv6 option (10-30% savings).

Description

This only adds the docs on how to be able to use k3sup to spin up IPv6-only clusters.

How Has This Been Tested?

A full cluster has been created using IPv6-only VPS with no issues.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.