Re-organize long docs into shorter ones

[milindsrivastava1997]

Some docs have grown long and hard to navigate. Split them into shorter, more focused docs.

[zzylol]

Related open work

• Cross-cutting documentation maintenance for design/tracking material such as Tracking issue for high-level TODOs for ASAPController #153 and Design inspirer links #158.
• It has no implementation dependency on those issues.

[zzylol]

Closing as completed by #298, which split the long ASAP-aware mapping document into focused documents, added a documentation index, and updated repository links.