Speedup sphinx-build using parallelization (GenericMappingTools#991)

Distribute sphinx-build over several processes in parallel
to make building on multiprocessor machines faster.
Using 'auto' which equals the number of CPUs available.
See docs at https://www.sphinx-doc.org/en/3.x/man/sphinx-build.html#cmdoption-sphinx-build-j.

* Move `-j auto` to SPHINXOPTS

Co-authored-by: Dongdong Tian <[email protected]>

Author: 2 people

doc/Makefile

@@ -1,7 +1,7 @@
 # Makefile for Sphinx documentation
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -j auto
 SPHINXBUILD   = sphinx-build
 SPHINXAUTOGEN = sphinx-autogen
 BUILDDIR      = _build

pygmt/tests/test_grdinfo.py

@@ -10,7 +10,7 @@
 
 def test_grdinfo():
     """
-    Make sure grd info works as expected.
+    Make sure grdinfo works as expected.
     """
     grid = load_earth_relief(registration="gridline")
     result = grdinfo(grid=grid, force_scan=0, per_column="n")