Add more space for the citations labels in HTML outputs

The citations labels of the bibliography are badly aligned for the html and
singlehtml output formats (see issue #128).

Add a custom CSS and use it to adjust the margin.

We visually tailor the setting for citations labels of 8 characters
maximum, which corresponds to the LaTeX formatter's setting.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

Author: vstehle

source/_static/custom.css

@@ -0,0 +1,4 @@
+/* We need a bit more space for our citations labels. */
+div.citation > p {
+  margin-left: 7em;
+}

source/conf.py

@@ -99,7 +99,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-# html_static_path = ['_static']
+html_static_path = ['_static']
 
 
 # -- Options for HTMLHelp output ------------------------------------------