feat: last updated time in docs (#913)

* Adding last updated time for docs

* Making file path general and other suggested changes

* Checking if time returned is null due to absence of git or some other issue

* Adding option to enable/disable update time feature and test-doc

* Adding simple unit tests for getGitUpdateTime()

* nits & rewrote failing test

* consistent test naming

* Adding optional updateEnableTime in documentation

* package-lock & yarn.lock

website/siteConfig.js

@@ -69,6 +69,7 @@ const siteConfig = {
   scrollToTopOptions: {
     zIndex: 100,
   },
+  enableUpdateTime: true,
 };
 
 module.exports = siteConfig;