doc: sphinx: Add sphinx-prompt

Sphinx-prompt[1] helps bring-in '.. prompt::' option that allows a
better rendered documentation, yet be able to copy paste without
picking up the prompt from rendered documentation.

[1] https://lore.kernel.org/all/87fs48rgto.fsf@baylibre.com/
Suggested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/sphinx/requirements.txt b/doc/sphinx/requirements.txt
index 4f411f7..6ccbe52 100644
--- a/doc/sphinx/requirements.txt
+++ b/doc/sphinx/requirements.txt
@@ -15,6 +15,7 @@
 six==1.16.0
 snowballstemmer==2.2.0
 Sphinx==3.4.3
+sphinx-prompt==1.5.0
 sphinx-rtd-theme==1.0.0
 sphinxcontrib-applehelp==1.0.2
 sphinxcontrib-devhelp==1.0.2