Improve reverse proxy documents and clarify the AppURL guessing behavior (#31003)

Fix #31002

1. Mention Make sure `Host` and `X-Fowarded-Proto` headers are correctly passed to Gitea
2. Clarify the basic requirements and move the "general configuration" to the top
3. Add a comment for the "container registry"
4. Use 1.21 behavior if the reverse proxy is not correctly configured

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

routers/api/packages/container/container.go

@@ -116,6 +116,8 @@ func apiErrorDefined(ctx *context.Context, err *namedError) {
 }
 
 func apiUnauthorizedError(ctx *context.Context) {
+	// TODO: it doesn't seem quite right but it doesn't really cause problem at the moment.
+	// container registry requires that the "/v2" must be in the root, so the sub-path in AppURL should be removed, ideally.
 	ctx.Resp.Header().Add("WWW-Authenticate", `Bearer realm="`+httplib.GuessCurrentAppURL(ctx)+`v2/token",service="container_registry",scope="*"`)
 	apiErrorDefined(ctx, errUnauthorized)
 }