doc: enable automatic parallelization

Sphinx supports parallelized build (-j) option. Setting it to `auto`
makes use of all available cores. This option seems to speed up the
build significantly on multi-core machines.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/CMakeLists.txt

@@ -59,7 +59,7 @@ if(${LATEXMK} STREQUAL LATEXMK-NOTFOUND)
 endif()
 
 if(NOT DEFINED SPHINXOPTS)
-  set(SPHINXOPTS -q)
+  set(SPHINXOPTS -q -j auto)
 else()
   separate_arguments(SPHINXOPTS)
 endif()