Fix stale src/ paths in docs and rewrite dead README sections

[anagainaru]

Docs-only cleanup: several paths still pointed at src/<subpackage>/ from before
the package moved under src/apeiron/. No Python imports were affected.

• Deployment READMEs (Frontier, Perlmutter): the source ./src/deployment/.../install_venv.sh
  and sbatch src/deployment/.../mnist_example.sbatch commands were broken as
  written — repointed to src/apeiron/deployment/....
• Test docstrings (6 files) and the header comment in examples/mnist/model.py:
  repointed to their real module paths.
• README.md: "What main.py Does", "Tuning Tips", and "Output" described a
  codebase that no longer exists — DummyCNN_MNIST, CL(), One_task_CL,
  update_CL_, n_epoch, and return_score appear in zero Python files in the
  repo. Rewrote all three against the actual src/main.py pipeline (config →
  logger → get_example harness → ContinuousMonitor → ContinuousTrainer on
  drift), with pointers to docs/architecture.md and docs/drift_detectors.md.

python -m src.main / src.drift_only / src.cl_only are unchanged — those
modules do live directly at src/. ruff check and ruff format --check pass.

Note the README rewrite is the one non-mechanical change here, so it's worth a reviewer's eye; the other nine files are pure path corrections.