Docs for SERVER-33000: Platform Support: add Ubuntu 18.04

Description:

This work adds support for a new platform, so the Production Notes and our Supported Platforms documentation will need updating.

Support starts in 4.1.1 for master, and we're aiming to add it for 4.0.1 for the v4.0 branch.

Scope of changes:

Impact to other docs outside of this product:

MVP:

Resources:

Engineering Ticket Description:

This can go to the backlog for now, but Ubuntu 18.04 will be available soon, so we should be ready to add this platform as soon as there are images out there.