enhanced staging sphinx documentation

@chapman39 is already working on this.

Since Jun 2, 2026.

https://github.com/llnl/blt/pull/764#discussion_r3321550467

```cmake
set(PROJECT_A_STAGE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_stage")
set(PROJECT_A_SOURCE_DIR "${PROJECT_A_STAGE_DIR}/src/docs")

add_custom_target(stage_my_docs
COMMAND ${CMAKE_COMMAND} -E remove_directory "${PROJECT_A_STAGE_DIR}"
COMMAND ${CMAKE_COMMAND} -E make_directory "${PROJECT_A_STAGE_DIR}"
COMMAND ${CMAKE_COMMAND} -E copy_directory "${PROJECT_B_SOURCE_DIR}/src" "${PROJECT_A_STAGE_DIR}/src"
COMMAND ${CMAKE_COMMAND} -E copy_directory "${PROJECT_B_SOURCE_DIR}/examples" "${PROJECT_A_STAGE_DIR}/examples"
COMMAND ${CMAKE_COMMAND} -E copy_directory "${PROJECT_B_SOURCE_DIR}/tests" "${PROJECT_A_STAGE_DIR}/tests"
COMMAND ${CMAKE_COMMAND} -E copy_directory "${CMAKE_CURRENT_SOURCE_DIR}" "${PROJECT_A_SOURCE_DIR}"
COMMENT "Staging documentation from project A and B sources"
VERBATIM)

blt_add_sphinx_target(my_docs
SOURCE_DIR "${PROJECT_A_SOURCE_DIR}"
CONF_DIR "${CMAKE_CURRENT_SOURCE_DIR}"
DEPENDS stage_my_docs)
```

the custom cmake target to stage the directories in to a single sphinx source dir, `stage_my_docs`, could be wrapped within the `blt_add_sphinx_target` macro.