Documentation: Add CSS to improve the look of equations

This comes from altMITgcm/MITgcm#12, which I
found in a link from
readthedocs/sphinx_rtd_theme#383.

This right-aligns the equation numbers, and only makes the link image
visible if your mouse is hovering above a given equation.

doc/source/_static/css/custom.css

@@ -0,0 +1,17 @@
+/* Make equation numbers float to the right */
+.eqno {
+    margin-left: 5px;
+    float: right;
+}
+/* Hide the link... */
+.math .headerlink {
+    display: none;
+    visibility: hidden;
+}
+/* ...unless the equation is hovered */
+.math:hover .headerlink {
+    display: inline-block;
+    visibility: visible;
+    /* Place link in margin and keep equation number aligned with boundary */
+    margin-right: -0.7em;
+}

doc/source/conf.py

@@ -180,3 +180,6 @@
                  'section': '%s',
                 }
 numfig_secnum_depth = 2
+
+def setup(app):
+    app.add_stylesheet('css/custom.css')