Guide on creating and including sound clips to be played by the i2s Speaker component. (#3718)

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* Update index.rst

missing link to the speaker component (this page is well hidden!) and added the link to the guide.

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Dots on i

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updated links

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xxd link for anything else but Linux...

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links in this format are strange.

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And again... links are weird.

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---------

Co-authored-by: H. Árkosi Róbert <robreg@zsurob.hu>

components/speaker/index.rst

@@ -92,4 +92,6 @@ Platforms
 See Also
 --------
 
+- :doc:`/guides/audio_clips_for_i2s`
+- :doc:`/components/speaker/i2s_audio`
 - :ghedit:`Edit`

guides/audio_clips_for_i2s.rst

@@ -0,0 +1,58 @@
+.. audio_clips_for_i2s:
+
+Create audio clip files for use with I²S Speakers
+=================================================
+
+It is possible to create sound clips to include in your build to use with I²S speakers. No need for a media player component!  
+
+- Using `Audacity <https://github.com/audacity/audacity>`__, convert audio to WAV, mono, 16kHz, Unsigned 8bit PCM
+
+.. figure:: /guides/images/save_as_wav.png
+    :alt: Audacity export dialog
+    :height: 200
+    :align: center
+
+- Convert again, this time with `SOX <https://github.com/chirlu/sox>`__.
+
+.. code-block:: console
+
+    sox startup.wav --bits 8 --encoding signed-integer --endian little startup_again.raw
+
+- Now convert it into a hexadecimal string using `xxd <https://github.com/ckormanyos/xxd>`__ into a C++ file.
+
+.. code-block:: console
+
+    xxd -i startup_again.raw startup.c
+
+- The resulting file needs a modification in the start line:
+  Open in an editor and change
+  ``unsigned char startup_again_raw[] = {…[SNIP]…}``
+  to
+  ``std::vector<unsigned char> startup_raw = {…[SNIP]…}``.
+
+Now you can rename the file to startup.h, put it inside the esphome configuration directory and put it in a include in your device config like this:
+
+.. code-block:: yaml
+
+    esphome:
+      includes:
+        - startup.h
+
+Now you can define using the audio clip using the following:
+
+.. code-block:: yaml
+
+    - speaker.play:
+        id: speaker
+        data: !lambda return startup_raw;
+
+Enjoy!
+
+HowTo by [NUT].
+
+See also
+--------
+
+- :doc:`/components/speaker/index`
+- :doc:`/components/speaker/i2s_audio`
+- :ghedit:`Edit`