Automatically generated hlt2 line reference documentation pages

Only displays a skeleton of the line configuration.

wishlist:

• expanded combiner details (cuts) and stats (rates, timings, and such)
• fix module docstrings to be Sphinx-friendly
• less ugly formatting

doc/selection/hlt2_lines_reference.rst

+=======================
+HLT2 lines reference
+=======================
+
+This page aims to be a comprehensive reference for all HLT2 lines.
+
+.. Please take note!
+.. The sections following this text are automatically generated!
+.. If you would like to change the behaviour of the autogeneration,
+.. please look instead at dump_hlt2_lines.py. 
+..
+.. Thanks for contributing!
+
+
+.. include:: hlt2_line_reference.generated.rst