improve docs code blocks highlighting (#2049)

Summary:
This PR adds missing Objective-C entry to the Docusaurus config, fixes Objective-C code blocks label and adds or replaces several code block labels to improve the currently highlighted blocks.

Prism in Docusaurus by default also includes syntax highlight for `jsx` and `tsx`, which improves the nodes and props highlight, so I have used those syntaxes in few places too.

I have also fixed one typo that I have spotted and my IDE made a cleanup of whitespaces in edited files.

## Changelog

* [website] improve docs code blocks highlighting

Pull Request resolved: https://github.com/facebook/flipper/pull/2049

Test Plan:
The changes have been tested running Flipper website on `localhost`.

## Preview

<img width="650" alt="Screenshot 2021-03-12 150934" src="https://user-images.githubusercontent.com/719641/110951135-fff20d00-8344-11eb-96db-1bdc82c8d5ea.png">

<img width="649" alt="Screenshot 2021-03-12 151022" src="https://user-images.githubusercontent.com/719641/110951268-2ca62480-8345-11eb-9d3b-1a48f1267776.png">

Reviewed By: priteshrnandgaonkar

Differential Revision: D27336599

Pulled By: passy

fbshipit-source-id: c2dfb3d8cad4675da0f5e1270cada1e56a0175c0

docs/getting-started/ios-native.mdx

@@ -91,7 +91,7 @@ AppDelegate.
 <Tabs defaultValue="ios" values={[{ label: 'iOS', value: 'ios'}, { label: 'Swift', value: 'swift'}]}>
 <TabItem value="ios">
 
-```objective-c
+```objc
 #import <FlipperKit/FlipperClient.h>
 #import <FlipperKitLayoutPlugin/FlipperKitLayoutPlugin.h>
 #import <FlipperKitLayoutComponentKitSupport/FlipperKitLayoutComponentKitSupport.h>

docs/getting-started/react-native-ios.mdx

@@ -107,7 +107,7 @@ The code below enables the following integrations:
 <Tabs defaultValue="ios" values={[{ label: 'iOS', value: 'ios'}, { label: 'Swift', value: 'swift'}]}>
 <TabItem value="ios">
 
-```objective-c
+```objc
 ...
 #if DEBUG
 #ifdef FB_SONARKIT_ENABLED