update example list

[zsdonghao]

Checklist

• I've tested that my changes are compatible with the latest version of Tensorflow.
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

• Reorder the example list
• Add ArcFace example

[DEKHTIARJonathan]

The documentation test is failing. Can you have a look why?
My implementation of the test might be faulty.

[lgarithm]

The log says

E               SphinxWarning: docs/user/example.rst:5:Duplicate explicit target name: "here".

you can preview docs/user/example.rst with ReStructured Text Previewer VSCode plugin:
(To install the plugin, just search rst in VScode Extensions Market.)

[zsdonghao]

@lgarithm thanks~