fix: set upstream in sync handler, cleanup docs around setting up github

quartz/cli/handlers.js

@@ -528,7 +528,7 @@ export async function handleSync(argv) {
   await popContentFolder(contentFolder)
   if (argv.push) {
     console.log("Pushing your changes")
-    const res = spawnSync("git", ["push", "-f", ORIGIN_NAME, QUARTZ_SOURCE_BRANCH], {
+    const res = spawnSync("git", ["push", "-uf", ORIGIN_NAME, QUARTZ_SOURCE_BRANCH], {
       stdio: "inherit",
     })
     if (res.status !== 0) {