Prevent corrections and completions in search field

In particular mobile browsers use correction mechanisms on input fields.
Search will often be done with technical terms or even class/method/etc
names that these correction mechanisms do not know, resulting in
unwanted changes. This PR deactivates these correction mechanisms.

I've also reported this upstream to sphinx at
sphinx-doc/sphinx#9307. See there for more
background.

Our current theme uses customized templates, so we have to adapt this
ourselves.

You can also that this is working by accessing the CI built docs with
a mobile device, and compare that with the released docs.

doc/_templates/layout.html

@@ -63,7 +63,7 @@
             <li><a href="{{ pathto('devel/index') }}">Contributing</a></li>
             <li class="nav-right">
                 <form class="search" action="{{ pathto('search') }}" method="get">
-                <input type="text" name="q" aria-labelledby="searchlabel" placeholder="Search"/>
+                <input type="text" name="q" aria-labelledby="searchlabel" placeholder="Search" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
                 </form>
             </li>
         </ul>

doc/_templates/search.html

@@ -17,7 +17,7 @@ <h1 id="search-documentation">{{ _('Search') }}</h1>
     "codex ellipse".
   </p>
   <form action="" method="get">
-    <input type="text" name="q" value="" />
+    <input type="text" name="q" value="" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
     <input type="submit" value="{{ _('search') }}" />
     <span id="search-progress" style="padding-left: 10px"></span>
   </form>