docs/tutorial: Replace coloring markup

This used to use doxygen's html blocks, which don't have a *direct*
equivalent in sphinx in code blocks.

Instead of adding this to the pygments highlighter, let's just use
some roles.

It's a teensy bit awkward as we then use block styling, but we want to
add more of our own styling anyway, so we can presumably get this
somehow, and these html tags look awkward and confuse people.

Fix #6640

[ci skip]

sphinx_doc_src/_static/pygments.css

@@ -79,3 +79,6 @@
 .highlight .il { color: #0000cf; font-weight: bold } /* Literal.Number.Integer.Long */
 .purple { color: #551a8b }
 .yellow { color: #FFFF00 }
+.red { color: #FF0000 }
+.gray { color: #555555 }
+.underline { text-decoration: underline }