doc: introduce how to build & run doxygen on ubuntu

Update readme doc to introduce how to build doxygen and run html
on Ubuntu machines.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

documentation/doxygen/readme.md

@@ -6,3 +6,43 @@
 4. Open the file ./Doxyfile 
 5.  To tab `Run` , Click `Run doxygen`
 
+# How to build & run doxygen html on Ubuntu
+
+The following steps are verified on Ubuntu 22.04：
+
+```shell
+$ lsb_release -a
+No LSB modules are available.
+Distributor ID:	Ubuntu
+Description:	Ubuntu 22.04.5 LTS
+Release:	22.04
+Codename:	jammy
+```
+
+The following packages (and dependents) need to be installed:
+
+```shell
+$ sudo apt update
+$ sudo apt install doxygen
+$ sudo apt install graphviz
+```
+
+Assume that the path of RT-Thead code tree is $RTT, execute the following command to build html.
+
+```shell
+$ cd $RTT/documentation/doxygen
+$ rm -rf html
+$ doxygen
+```
+
+A new html directory will be created and all the html files will be placed in this directory.
+
+If you want to quickly browse HTML locally (in Ubuntu environment), you can enter the html directory and start a local HTML server through Python.
+
+```shell
+$ cd html
+$ python3 -m http.server
+Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
+```
+
+Open the browser and enter `http://<IP>:8000/index.html` to access the created html web pages. If it is a local access, then `<IP>` should be replaced by `localhost`. If it is a remote access, then `<IP>` should be replaced by the actual accessible IP address of the machine where HTML is located.