Define markdown convention/formatting rules

It would be good to define our standards/expectations for markdown formatting somewhere consistently.

A good place for this would be `CONTRIBUTING.md`, but other places could work too.

In addition to introducing documentation on how markdown should be written in the repo, we should probably try and
implement some tooling that enables machines to check our output for us:

- https://github.com/hukkin/mdformat
- https://github.com/DavidAnson/markdownlint
- https://dprint.dev/overview/ w/ https://github.com/dprint/dprint-plugin-markdown
- Prettier (listed last because it's intentionally hard to configure)

I think a good PR that resolves this issue would have a comparison of the output of these tools on one of our files, along with integration of the tool into CI so that we can catch issues going forward.

从 CONTRIBUTING.md 和仓库中一个具有代表性的 Markdown 文件开始。按照提议比较 mdformat、markdownlint、带有其 Markdown 插件的 dprint 和 Prettier，然后记录所选的约定，并将选定的检查集成到 CI 中，以便自动检查今后的 Markdown 输出。

由索引模型根据 Issue 内容生成。