DOC: Fix indentation of lists in rst files

[datapythonista]

The indentation for lists in the restructuredText standard is next:

• No indentation for top level lists
• 4 space indentation for sublists

When a top list is indented (with for example one space), the list is included in a blockquote element when rendered as html. In the current sphinx template, this does not seem to have any visual impact, but with other templates (e.g. the sphinx-responsive-theme) it does. See this screenshot for an example:

https://user-images.githubusercontent.com/10058240/41320132-6d1b1078-6e96-11e8-9b7b-a6617bdcdd16.png

Regarding sublists, sphinx is flexible when applying the standard, and sublists indented with for example 3 spaces, are rendered correctly as sublists. But other software like pandoc (not currently used to render rst files), does not consider the indented lists as sublists unless they are indented with the right number of spaces (four).

Ideally we want to have all the top level lists without indentation, and the sublists with 4 space indentation, so we can replace the current sphinx template, and we can render the rst files with software that is strict with the standard.