Update Project Isolation documentation page

[bamboo]

• Add information about our current plans, known caveats, get rid of mixed terminology, etc
• Move it together with the rest of the documentation (without linking to it from anywhere just yet)

[ljacomet]

Moved to 8.10 RC1.

If this was delivered with 8.9, please close and update the milestone.

[alllex]

We need to make sure the updated page contains the words "project isolation" verbatim.

Even though we decided to go with the "Isolated Projects" name for the feature, folks outside Gradle who are aware of it keep calling it "project isolation". To me this is a signal we'll never get away from people referring to the feature like this.

We did discuss that "project isolation" is something the users will do to make their build IP compatible. So it can fit naturally in one of the first paragraphs on the page.

FWIW, search engines today show the right result for googling "gradle project isolation", but I'd like us to have the alternative spelling in clear text.

For the record, our own in-docs search shows nothing relevant with the "project isolation" lookup