docs/tutorial: Fix prompt display

This displays a colored prompt, which we emulate by adding explicit
roles that are translated to css classes.

For other things, like "eror" this might not be enough because those
often need the rest of the line to still be styled, and I've not found
a way to add some explicit styling to a code block.

See #5696.

[ci skip]

sphinx_doc_src/_static/pygments.css

@@ -77,3 +77,5 @@
 .highlight .vi { color: #000000 } /* Name.Variable.Instance */
 .highlight .vm { color: #000000 } /* Name.Variable.Magic */
 .highlight .il { color: #0000cf; font-weight: bold } /* Literal.Number.Integer.Long */
+.purple { color: #551a8b }
+.yellow { color: #FFFF00 }

sphinx_doc_src/tutorial.rst

@@ -567,6 +567,9 @@ You can define your own prompt::
     New Prompt % 
 
 
+.. role:: purple
+.. role:: yellow
+
 Multiple lines are OK. Colors can be set via ``set_color``, passing it named ANSI colors, or hex RGB values::
 
     >_ function fish_prompt
@@ -575,9 +578,11 @@ Multiple lines are OK. Colors can be set via ``set_color``, passing it named ANS
           set_color FF0
           echo (pwd) '>' (set_color normal)
       end
-    <span style="color: purple">02/06/13</span>
-    <span style="color: #FF0">/home/tutorial ></span>___
 
+will look like
+
+| :purple:`02/06/13`
+| :yellow:`/home/tutorial >`
 
 You can choose among some sample prompts by running ``fish_config prompt``. ``fish`` also supports RPROMPT through ``fish_right_prompt``.