add sparse url in cargo package guide (#26937)

Hello,

The current package guide for cargo gives you only the git index, with
the HTTP Index stabilized being used as default for crates.io and being
better for most use-cases.

However, it's not documented that gitea supports the sparse spec, and it
does not require the _crates-index git repo for the sparse api.

I personally think we should push users to use the sparse instead of the
git repository. (Even let users disable crates-index repos if they only
want to use sparse)

docs/content/usage/packages/cargo.zh-cn.md

@@ -50,7 +50,8 @@ Cargo 将可用软件包的信息存储在一个存储在 git 仓库中的软件
 default = "gitea"
 
 [registries.gitea]
-index = "https://gitea.example.com/{owner}/_cargo-index.git"
+index = "sparse+https://gitea.example.com/api/packages/{owner}/cargo/" # Sparse index
+# index = "https://gitea.example.com/{owner}/_cargo-index.git" # Git
 
 [net]
 git-fetch-with-cli = true