Add metadata constants to scripts/MetadataMd (#1131)

* Add metadata constants to scripts/MetadataMd

* Use name instead of prefix for Extrinsics

Since the js api uses the name and not the prefix (api.tx.name.xx)
and finality_tracker doesn't have a prefix

* Lint

docs/.vuepress/config.js

@@ -45,6 +45,7 @@ module.exports = {
         sidebarDepth: 0,
         children: [
           ['/METHODS_RPC.md', 'Substrate RPC'],
+          ['/METHODS_CONSTANTS.md', 'Constants (defaults)'],
           ['/METHODS_STORAGE.md', 'State storage (defaults)'],
           ['/METHODS_EXTRINSICS.md', 'Extrinsics (defaults)'],
           ['/METHODS_EVENTS.md', 'System events (defaults)']