minor symfony#9972 Use the ReadTheDocs theme as the default Sphinx theme (javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
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Use the ReadTheDocs theme as the default Sphinx theme

I can't make our Sphinx theme work and I can't fix any of the issues reported about it. So let's stop wasting time on this and let's use the default Sphinx theme provided by ReadTheDocs (https://github.com/rtfd/sphinx_rtd_theme).

It doesn't look like symfony.com, but it's beautiful, responsive and it works. In the future we can use it as the base to create our own theme.

This is how it looks after running `cd _build && make html`:

![index](https://user-images.githubusercontent.com/73419/41984189-5b998672-7a30-11e8-83f5-f6fdeb437a44.png)

![page](https://user-images.githubusercontent.com/73419/41984192-5d43f606-7a30-11e8-9db4-366ac6fdafec.png)

Commits
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a5a2171 Use the ReadTheDocs theme as the default Sphinx theme

Author: javiereguiluz

_build/_theme/_templates/globaltoc.html

@@ -26,17 +26,14 @@ def __init__(self, builder, *args, **kwds):
         self.highlightlinenothreshold = 0
 
     def visit_literal(self, node):
-        self.body.append(self.starttag(node, 'tt', '', CLASS='docutils literal'))
-        self.body.append('<code>')
+        self.body.append(self.starttag(node, 'code', '', CLASS='docutils literal notranslate'))
 
     def depart_literal(self, node):
         self.body.append('</code>')
-        self.body.append('</tt>')
 
     def visit_admonition(self, node, name=''):
         self.body.append(self.starttag(node, 'div', CLASS=('admonition-wrapper')))
-        self.body.append('<div class="' + name + '"></div>')
-        self.body.append('<div class="admonition admonition-' + name + '">')
+        self.body.append('<div class="admonition ' + name + '">')
         if name and name != 'seealso':
             node.insert(0, nodes.title(name, customadmonitionlabels[name]))
         self.set_first_last(node)
@@ -46,7 +43,6 @@ def depart_admonition(self, node=None):
 
     def visit_sidebar(self, node):
         self.body.append(self.starttag(node, 'div', CLASS=('admonition-wrapper')))
-        self.body.append('<div class="sidebar"></div>')
         self.body.append('<div class="admonition admonition-sidebar">')
         self.set_first_last(node)
         self.in_sidebar = 1