pw_hdlc_lite: Added module for HDLC transport layer

The CL contains a skeleton for the hdlc module and a stream writer for
sys_io.

Change-Id: I4633deee1e049ebd21d552cca3962c77ea093d0f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/13480
Commit-Queue: Shane Gomindes <shaneajg@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_hdlc_lite/docs.rst b/pw_hdlc_lite/docs.rst
new file mode 100644
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+.. _chapter-pw-hdlc:
+
+.. default-domain:: cpp
+
+.. highlight:: sh
+
+------------
+pw_hdlc_lite
+------------
+pw_hdlc_lite is a module that enables serial communication between devices
+using the HDLC-Lite protocol.
+
+Compatibility
+=============
+C++17
+
+Dependencies
+============
+* ``pw_preprocessor``
+* ``pw_status``
+* ``pw_span``
+* ``pw_sys_io``
+* ``pw_stream``
+
+Features
+========
+
+pw::stream::SerialWriter
+------------------------
+The ``SerialWriter`` class implements the ``Writer`` interface by using sys_io
+to write data over a communication channel.
+
+
+Future work
+^^^^^^^^^^^
+- Adding the code for the Encoder and Decoder.