Detail roadmap for changing to SWDM v4 as default

[fershad]

The purpose of this issue is to track the process of transitioning to using the Sustainable Web Design Model (SWDM) version 4 as the default model in CO2.js.

Currently, SWDM is the default model used by the library. Developers have the option to switch models (see docs) between SWDM and OneByte. With the implementation of SWDM v4 developers also have the option to now switch between different version of SWMD (see #208). The default SWDM version used by CO2.js is version 3, and developers have to opt-in to switching to the newer version 4 methodology.

In the same way as we previously switched the default model from OneByte to SWDM (see #100) we would also want to eventually update the library to use SWDM version 4 as the default methodology.

While it is not expected that this would be a breaking change, we would still approach this very gradually and communicate it with developers well ahead of time. This issue tracks those tasks, as well as provides a place for conversation about the switch to the newer methodology.

Tasks:

• [DECISION] Decide on a date/version in which SWDM version 4 will be set as the default
• [DECISION] Will SWDM version 3 be removed from CO2.js or simply left for developers to "opt-in" to using?
• Add warning to CO2.js when developers use SWDM version 3
• Update documentation site to reflect this information
• Update the CO2.js page on the Green Web Foundation site to reflect this information
• Update the CO2.js quick start guide to reflect this information
• Publish announcements through regular release communication channels (LinkedIn, Slack)
• Prepare Pull Request for change
• Merge change
• Publish regular release communication to notify of change

Additional context (if applicable)
Related to PR: #208
Related to Issue: #119