René Paris authored Jun 25, 2021 and Reinhard Biegel committed Jul 08, 2021

- Sphinx documentation moved from `sim/doc/source` to `doc/sphinx`
- CMakeLists adjusted accordingly

- Added placeholders for referencing files outside of `doc/sphinx`
  - @OP_REL_ROOT@ -> starting from folder `.`
  - @OP_REL_GUI@  -> starting from folder `gui`
  - @OP_REL_SIM@  -> starting from folder `sim` (as before)

- Examples for placeholder usage in sphinx:
  - `.. literalinclude:: @OP_REL_ROOT@/.clang-format`
  - `.. literalinclude:: @OP_REL_GUI@/application/openPASS.cpp`
  - `.. literalinclude:: @OP_REL_SIM@/src/core/CMakeLists.txt`

Signed-off-by: Rene Paris <rene.paris@in-tech.com>