doc: Add manpage for feedback themes

This should direct users to the documentation.

Prompted by https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1050762

doc/feedback-themes.rst

@@ -0,0 +1,32 @@
+.. _feedback-themes(5):
+
+===============
+feedback themes
+===============
+
+---------------------------------
+Theme configuration for feedbackd
+---------------------------------
+
+DESCRIPTION
+===========
+
+In order to reflect device specifics and user overrides the feedback
+(such as sound, haptic or led) that are being triggered by events can
+be configured. This allows for device as well as user specific
+configuration. Themes use a json format and can be validated by
+``fbd-theme-validate``.
+
+For details on how to create or modify feedback themes see [feedbackd's documentation](
+https://source.puri.sm/Librem5/feedbackd#feedback-theme)
+
+Sound events
+============
+
+Sound feedbacks specify an event name from a XDG sound theme. Sound themes
+are described in the [Sound theme spec](https://freedesktop.org/wiki/Specifications/sound-theme-spec/).
+
+See also
+========
+
+``feedbackd(8)`` ``fbcli(1)`` ``fbd-theme-validate(1)``