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
remove long syntax from the user's guide
Merge branch 'theory-docs'
docs: simplify paragraph in doc/index.rst
docs: simplify doc/introduction.rst
docs: move doc/examples/using_salome.rst -> doc/using_salome.rst

- doc/examples/ contents is fully auto-generated
docs: put Primer under Examples
docs: fix doc/index.rst for moved IGA section, reference tutorial
new doc page with long syntax
tutorial with short syntax of keywords, closes #272
Merge pull request #275 from vlukes/short_syntax
fix image path