doc: update document

- add hint for gray virtools import export button
- add outcome of leaving blank ballance texture folder setting.
- update compile chapter according to the change of build scripts.

.gitattributes

@@ -3,4 +3,4 @@
 # Element placeholder mesh should be save as binary
 *.ph binary
 # Raw json data should be binary, although i edit it manually
-assets/jsons/*.json    binary
+assets/jsons/*.json5    binary