docs: rewrite IMPLEMENTING_JRES.md for BaseJRE pattern

PR #1288 refactored all standard JREs to embed BaseJRE (14 lines each),
but the guide still showed the old 100+ line manual implementation with
explicit Supply()/Finalize() boilerplate.

Rewrite to document the actual pattern: newBaseJRE() + optional
extraFinalizeOpts. Removes sections on manual memory calculator and
JVMKill integration (now internal to BaseJRE). Adds variation-point
table and examples for all three patterns (standard, exact-dir, error-hint).

Author: stokpop