feat: custom index.html should be optional in project doc root

There are two possible documentation structure design:
 - submodules included inside the root doc folder (preferred)
 - submodules in their own doc folder and a separate sphinx target

The custom index.html is really useful only in the second situation.
Therefore it is now copied to the output dir only if it is present in
the source dir.

cmake/SphinxGeneration.cmake

@@ -90,27 +90,26 @@ if(SPHINX_FOUND)
     # invoked.
     set_target_properties(sphinx PROPERTIES EXCLUDE_FROM_ALL TRUE)
 
-    # Add a target for copying the index.html from the doc dir to the sphinx
-    # build dir. A dependency on this target will be added to the master sphinx
-    # target.
-    add_custom_command(
-      OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/sphinx/index.html
-      COMMAND
-        ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/doc/index.html
-        ${CMAKE_CURRENT_BINARY_DIR}/sphinx/index.html
-      DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/doc/index.html)
-    add_custom_target(copy_doc_index ALL
-                      DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/sphinx/index.html)
-
     # Setup sphinx doc master target and add other submodules as dependencies
     function(_add_master_sphinx_target)
       _master_sphinx_target()
       asap_with_sphinx(${master_sphinx_target})
-      add_dependencies(
-        ${master_sphinx_target}_sphinx copy_doc_index
-        # Add more submodule documentation targets after this, using variables
-        # in the target names consistently with the module's CMakeLists.txt.
-      )
+
+      set(index_file_src "${CMAKE_CURRENT_SOURCE_DIR}/doc/index.html")
+      set(index_file_out "${CMAKE_CURRENT_BINARY_DIR}/sphinx/index.html")
+      if(EXISTS ${index_file_src})
+        message(
+          STATUS "Will use project custom doc index file: ${index_file_src}")
+        # Add a target for copying the index.html from the doc dir to the sphinx
+        # build dir. A dependency on this target will be added to the master
+        # sphinx target.
+        add_custom_command(
+          OUTPUT ${index_file_out}
+          COMMAND ${CMAKE_COMMAND} -E copy ${index_file_src} ${index_file_out}
+          DEPENDS ${index_file_src})
+        add_custom_target(copy_doc_index ALL DEPENDS ${index_file_out})
+        add_dependencies(${master_sphinx_target}_sphinx copy_doc_index)
+      endif()
       add_dependencies(sphinx ${master_sphinx_target}_sphinx)
     endfunction()
     _add_master_sphinx_target()