[Brigadier] Add a simple example

查看现有的 Brigadier 文档页面，并选择一个能够演示权限、嵌套参数或字面量、复用的执行器以及游戏内可见变化的命令示例。创建一份面向初学者的分步指南，为每个概念链接到详细页面；当指南展示出一个完整、易于用户使用的命令，且不需要预先了解 Brigadier 时，即视为完成。

[!NOTE]
This issue is part of a series of issues regarding the extension of the Paper documentation regarding Paper's Brigadier API.

Currently, the documentation documents the separate aspects of Brigadier, but doesn't really bring all of them together in a real command. Therefore, it would be great if the documentation had a step-by-step creation of an actual user-friendly and refined command which provides some more logic. The command should display the usage of the following aspects:

• Permissions (using requirements)
• Nested arguments/literals
• Reused command executors (E.g. default value if an argument is not provided)
• Some visible change in-game

An example could be a skull-giving command, which has the following structure:

/skull
/skull <offline-player>
/skull <offline-player> <amount>

[!WARNING]
The command example really isn't a great one. I would appreciate it if somebody who has a better idea could suggest it here

It should be made sure that this guide helps even a beginner go through the process of building up a command, but it doesn't have to go into detail of what the separate steps mean precisely. For this, it can just be linked to their respective pages where they are described in more detail.