[Future] Document new analyzer plugin system

[MaryaBelanger]

New design that allows users to extend the functionality of the analysis server, making it so it doesn't require so much memory to run multiple plugins.

User-facing behavior change (what needs to be documented), from the design doc:

Developers can specify that a package (or a set of packages, like in a mono-repo) is to be analyzed with a set of analyzer plugins by specifying pub package names and version constraints in an analysis_options.yaml file.

In the plugins2 (name likely to change) section of the file, developers can include a simple list of analyzer plugin package names, each mapped to a version constraint. This looks exactly like the pubspec dependencies map.

Also document some context, why users might want to do this and who it's useful for.

Where: Something like a sibling page to https://dart.dev/tools/analysis, but much shorter.