docs: add architecture overview for bootstrapper subpackage

[LalatenduMohanty]

The bootstrapper was refactored from a monolithic 2278-line file into a 14-module subpackage in #1226. There is no architecture documentation describing the module layout, phase pipeline, or class hierarchy.

An architecture doc would help contributors understand:

• The bootstrap phase flow (RESOLVE → START → PREPARE_SOURCE → ... → COMPLETE)
• The Phase ABC and its 7 concrete subclasses
• How WorkItem accumulates state across phases
• The thread pool and background I/O model
• The module structure under src/fromager/bootstrapper/

Suggested location: docs/concepts/bootstrapper-architecture.rst