Create distinct _build/.gitignore

.gitignore

@@ -14,8 +14,3 @@ desktop.ini
 /composer.lock
 /composer.phar
 /vendor
-
-# Build system
-/_build/phpdoc
-/_build/phpdoc.cache
-/_build/deploy-*.git

_build/.gitignore

@@ -0,0 +1,4 @@
+# Ignore temporary directories created while building/deploying Pico
+/phpdoc
+/phpdoc.cache
+/deploy-*.git