Reorganize documentation for progressive disclosure

[iskandr]

What changed

• replace the opening score-direction table with a concise explanation of the
  three prediction tasks
• move score interpretation after the first prediction
• lead CLI and Python tutorials with the common user path
• put the runnable training example before custom data schemas
• put automatic hardware behavior before environment overrides
• move test-suite internals behind the actual testing commands
• clearly label generated CLI and API pages as exhaustive references
• bump the package version to 2.3.0rc19

Why

New readers were seeing low-level output and configuration details before they
had a mental model of what MHCflurry does or which workflow to choose. The
revised order uses progressive disclosure while preserving detailed material
later in each guide.

Validation

• ./lint.sh
• make -C docs generate html SPHINXOPTS=-W
• make -C docs doctest SPHINXOPTS=-W (20 passed)
• python -m pytest -q test/test_packaging.py (2 passed)
• python -m pytest test/ (938 passed)