Reference documentation for types is a bit odd

[ethomson]

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