Refactor overview to add scope, first sketch for memory.map

[dtig]

This PR is a first step at refactoring the overview document.

• Clearly define the current scope of the proposal
• Factor the overview better, and add a first sketch for memory.map
• Add TODOs as needed
• Delete approaches not being currently considered
• Delete open questions as they're not reflective - will follow up with more detailed github issues

[dtig]

Explicitly cc @bvisness for review as we figure out permissions to add as reviewer.

[dtig]

Merging to have something current that we can iterate on.