cmake add_custom_command is only run if any of the listed outputs is missing
or if any of the dependencies are out of date. If any of the documentation
.rst files is changed this didn't cause the custom command to run.
I've changed this so the command always runs by adding an extra dummy output
file that doesn't ever get created. The sphinx builder has some incremental
build functionality inside it so this will identify whether any .rst files
have changed
Note - I also had to set the cmake source file property to SYMBOLIC to avoid the
following Visual Studio warning which was added in Visual Studio 2019 (v16.4):

warning MSB8065: Custom build for item XXXX succeeded, but specified output
"c:\mantid\build\docs\dummyoutput_alwaysrun" has not been created. This may cause
 incremental build to work incorrectly.