fix: drop --locked from VCS PKGBUILDs, add setup and troubleshooting docs

--locked is unreliable for -git packages where Cargo.lock may diverge
from the latest source. Also documents the required paru -Sy --pkgbuilds
step and common error resolutions.

moongreet-git/PKGBUILD

@@ -32,7 +32,7 @@ pkgver() {
 
 build() {
     cd "$srcdir/greetd-moongreet"
-    cargo build --release --locked
+    cargo build --release
 }
 
 package() {