[doc] Add dftkit python path for sphinx build

The converters module now imports from triqs_dftkit, which
requires its build python directory on the PYTHONPATH.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Wentzell

doc/CMakeLists.txt

@@ -13,7 +13,7 @@ add_custom_target(${PROJECT_NAME}_docs_sphinx ALL)
 add_custom_command(
   TARGET ${PROJECT_NAME}_docs_sphinx
   POST_BUILD
-  COMMAND PYTHONPATH=${PROJECT_BINARY_DIR}/python:$ENV{PYTHONPATH} ${SPHINXBUILD_EXECUTABLE} -j auto -c . -b html ${CMAKE_CURRENT_SOURCE_DIR} html
+  COMMAND PYTHONPATH=${PROJECT_BINARY_DIR}/python:${triqs_dftkit_BINARY_DIR}/python:$ENV{PYTHONPATH} ${SPHINXBUILD_EXECUTABLE} -j auto -c . -b html ${CMAKE_CURRENT_SOURCE_DIR} html
 )
 
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