Cross reference links can lead to 404 pages

The latest version of the program supports cross-references like `[ClassName.method]`, etc. but the links don't always work. Part of the problem cannot be fixed without input from the user.

## The problem

When previewing markdown documents on your desktop, the links should point to markdown files to work, as you're browsing through your file system. E.g. `ClassName.md`

If you convert to html and/or put the files on the web, the links should point to the page's name or file, `./ClassName` or `./ClassName.html` or the lowercase equivalents.

Hugo may change the case of the links depending on the website's settings I believe?

## Solution

For markdown, I'm currently linking to `../PageName`. This *should* work if you put your pages online with a CMS, but it won't if you output your files as html directly. To solve this, we might need to have an html output that writes links as `../pagename.html#header-name`. We can look at how Sphinx and the Godot docs handle that.

In the hugo format, links should probably use the `{{< relref >}}` shortcode. This shortcode takes a relative path to a markdown document and resolves the final path for you. We have the exact name of the markdown documents so this would work reliably.

ソースファイルもテストも指定されていません。Markdown、HTML、Hugo 出力の相互参照リンク生成のエントリポイントを特定し、ここで説明されている想定パスを Sphinx および Godot のドキュメントの動作と比較してください。生成されたリンクがデスクトップ Markdown、直接 HTML、Hugo サイトで機能すれば完了です。

索引モデルが issue の本文から書いたものです。