docs: new doc/ebcs_implementation.rst - explanation of EBCs implementation
docs: move IGA section from tutorial to user's guide
docs: new doc/theory.rst

- move notes on solving PDEs from doc/tutorial.rst to new
  doc/solving_pdes_by_fem.rst
- add doc/ebcs_implementation.rst to doc/theory.rst
- update doc/index.rst
remove doc/notes.rst
Merge branch 'theory-docs'