Explanation for using gitea's variables in `.tmpl` files. Thanks to @wxiaoguang for advising me on [discord](https://discord.com/channels/322538954119184384/561007778139734027/1141217820441587722).
Related to https://github.com/go-gitea/gitea/issues/25503 --------- Co-authored-by: delvh <dev.lh@web.de>