3.18 Technical documentation

[wordpressfan]

Is your feature request related to a problem? Please describe.
We need to share the knowledge about 3.18 features with the support team internally before going live.

Describe the solution you'd like
Create some notion docs describing the technical details about Host google fonts locally, DJE safe mode, DJE setting structure, and any other new code change.

Additional context
This was mentioned in today's spring planning meeting.

https://www.notion.so/wpmedia/Self-host-Google-Fonts-15aed22a22f080a788fcc0d4377dbe23

[wordpressfan]

I did a small update to the notion document in the part of How does it interact with our other options? to mention the interaction with RUCSS.

I think we are good for Host google fonts locally feature, @piotrbak do u know if we have documentation for the remaining features like DJE safe mode? or we should make this?