Add document for rules embed in package

[jingkaimori]

Is your feature request related to a problem? Please describe.

xmake-io/xmake#2374 describes how to embed rules in package, but current document about this part is missing.

Describe the solution you'd like

Fix format issues in existing document

Describe alternatives you've considered

No response

Additional context

No response

[waruqi]

https://xmake.io/#/zh-cn/package/remote_package?id=%e5%88%86%e5%8f%91%e5%92%8c%e4%bd%bf%e7%94%a8%e8%87%aa%e5%ae%9a%e4%b9%89%e5%8c%85%e8%a7%84%e5%88%99

[waruqi]

https://xmake.io/#/package/remote_package?id=distributing-and-using-custom-package-rules

[jingkaimori]

some format issue are seen

[waruqi]

what issues? you can open a pr to improve it.