docs: add build cascade analysis and modernization plan

Add two analysis documents produced during the investigation of
cascading builder re-invocations:

- BUILD-CASCADE-ANALYSIS.md: Root cause analysis of why editing
  unrelated projects triggers full builds of DSL projects. Traces
  the issue to a vestigial builder.needRebuild() call from 2016.

- DDK-BUILDER-MODERNIZATION.md: Inventory of deprecated and
  unimplemented Xtext APIs in the DDK builder infrastructure,
  with a phased plan for modernization.

Author: joaodinissf