Reference documentation for types is a bit odd

There are a few examples where we expose the type details in sys but also have it opaque in the regular public docs. For example, git_credential_username.

In these cases, we don't provide any information about the type in the standard credential docs, only in sys. We should do better.

I would expect to see something here: https://libgit2.org/docs/reference/main/credential/index.html

Instead, I only see it here: https://libgit2.org/docs/reference/main/sys/credential/index.html

When we fix this, we need to update the redirects in libgit2.org/libgit2/index.html to account for it

libgit2.org/docs/reference/main/credential/index.html の標準 credential リファレンスと sys リファレンスを、git_credential_username を例として比較します。次に libgit2.org/libgit2/index.html を調べ、リファレンスドキュメントと併せてリダイレクトを更新します。関連する型情報が通常の credential ドキュメントに現れ、リダイレクトが変更を反映していれば完了です。