Update CMake-based doc build

  set(VIRTUALENV ${PYTHON_EXECUTABLE} -m virtualenv)

  file(GLOB DOC_SOURCES ${LAMMPS_DOC_DIR}/src/[^.]*.txt)

  file(GLOB PDF_EXTRA_SOURCES ${LAMMPS_DOC_DIR}/src/lammps_commands*.txt ${LAMMPS_DOC_DIR}/src/lammps_support.txt ${LAMMPS_DOC_DIR}/src/lammps_tutorials.txt)

  list(REMOVE_ITEM DOC_SOURCES ${PDF_EXTRA_SOURCES})

  file(GLOB DOC_SOURCES ${LAMMPS_DOC_DIR}/src/[^.]*.rst)

  add_custom_command(

    OUTPUT docenv

    COMMAND ${DOCENV_BINARY_DIR}/pip install --upgrade ${LAMMPS_DOC_DIR}/utils/converters

  )

  set(RST_FILES "")

  set(RST_DIR ${CMAKE_BINARY_DIR}/rst)

  file(MAKE_DIRECTORY ${RST_DIR})

  foreach(TXT_FILE ${DOC_SOURCES})

    get_filename_component(FILENAME ${TXT_FILE} NAME_WE)

    set(RST_FILE ${RST_DIR}/${FILENAME}.rst)

    list(APPEND RST_FILES ${RST_FILE})

    add_custom_command(

      OUTPUT ${RST_FILE}

      DEPENDS requirements.txt docenv ${TXT_FILE}

      COMMAND ${DOCENV_BINARY_DIR}/txt2rst -o ${RST_DIR} ${TXT_FILE}

    )

  endforeach()

  add_custom_command(

    OUTPUT html

    DEPENDS ${RST_FILES}

    COMMAND ${CMAKE_COMMAND} -E copy_directory ${LAMMPS_DOC_DIR}/src ${RST_DIR}

    COMMAND ${DOCENV_BINARY_DIR}/sphinx-build -j ${NPROCS} -b html -c ${LAMMPS_DOC_DIR}/utils/sphinx-config -d ${CMAKE_BINARY_DIR}/doctrees ${RST_DIR} html

    DEPENDS ${DOC_SOURCES} docenv requirements.txt

    COMMAND ${DOCENV_BINARY_DIR}/sphinx-build -j ${NPROCS} -b html -c ${LAMMPS_DOC_DIR}/utils/sphinx-config -d ${CMAKE_BINARY_DIR}/doctrees ${LAMMPS_DOC_DIR}/src html

  )

  add_custom_target(