readme: add documentation details

Inform the users on how to quickly build the documentation.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

README.md

@@ -21,6 +21,7 @@ applications. Some of the features demonstrated in this example are:
 - Out-of-tree libraries
 - Example CI configuration (using Github Actions)
 - Custom [west extension][west_ext]
+- Doxygen and Sphinx documentation boilerplate
 
 This repository is versioned together with the [Zephyr main tree][zephyr]. This
 means that every time that Zephyr is tagged, this repository is tagged as well
@@ -94,3 +95,36 @@ To execute Twister integration tests, run the following command:
 ```shell
 west twister -T tests --integration
 ```
+
+### Documentation
+
+A minimal documentation setup is provided for Doxygen and Sphinx. To build the
+documentation first change to the ``doc`` folder:
+
+```shell
+cd doc
+```
+
+Before continueing, check if you have Doxygen installed. It is recommended to
+use the same Doxygen version used in [CI](.github/workflows/docs.yml). To
+install Sphinx, make sure you have a Python installation in place and run:
+
+```shell
+pip install -r requirements.txt
+```
+
+API documentation (Doxygen) can be built using the following command:
+
+```shell
+doxygen
+```
+
+The output will be stored in the ``_build_doxygen`` folder. Similarly, the
+Sphinx documentation (HTML) can be built using the following command:
+
+```shell
+make html
+```
+
+The output will be stored in the ``_build_sphinx`` folder. You may check for
+other output formats other than HTML by running ``make help``.