Add PDF appendix support to scenario-include directive

The scenario-include directive now has two modes:

HTML (unchanged):
  Scenarios are rendered inline as collapsible <details> blocks,
  exactly as before.

PDF / LaTeX:
  Scenarios are moved to an auto-generated appendix page grouped by
  dfetch command (update, check, add, …). In the place of the directive
  the text shows a cross-reference: "Scenarios: see <Feature title> in
  the appendix." The command grouping is derived from the @command
  behave tag added to each feature file in the previous commit.

New directive option :inline: keeps a scenario in its original
position even in PDF mode, for cases where the example is essential
reading in context:

    .. scenario-include:: ../features/journey-basic-usage.feature
       :inline:

New directive scenario-appendix renders the collected scenarios in
the PDF appendix; in HTML it emits an orientation note instead.

doc/appendix/scenarios.rst added as the appendix landing page; it is
wired into the index.rst toctree under a new "Appendix" caption.

https://claude.ai/code/session_01BvyikyxX9c3sDXev8HdP4f

Author: claude