Fix website syntax highlighting

Summary:
We have a bunch of code blocks using languages that aren't rendered by default, so adding syntax highlighting for those languages.

Also, some uses are using the wrong "name" for the language, so I've fixed them.

Reviewed By: mweststrate

Differential Revision: D21974341

fbshipit-source-id: 17146ea5ae4979241c51b3707035470e9742a104

docs/extending/create-plugin.mdx

@@ -52,7 +52,7 @@ public class MyFlipperPlugin implements FlipperPlugin {
 @end
 ```
 <!--C++-->
-```c++
+```cpp
 class MyFlipperPlugin : public FlipperPlugin {
 public:
   std::string identifier() const override { return "MyFlipperPlugin"; }
@@ -130,7 +130,7 @@ connection.receive("getData", new FlipperReceiver() {
 @end
 ```
 <!--C++-->
-```c++
+```cpp
 void MyFlipperPlugin::didConnect(std::shared_ptr<FlipperConnection> conn) {
   conn->receive("getData", [](const folly::dynamic &params,
                              std::unique_ptr<FlipperResponder> responder) {
@@ -177,7 +177,7 @@ connection.send("MyMessage",
 [connection send:@"getData" withParams:@{@"message":@"hello"}];
 ```
 <!--C++-->
-```c++
+```cpp
 void MyFlipperPlugin::didConnect(std::shared_ptr<FlipperConnection> conn) {
   dynamic message = folly::dynamic::object("message", "hello");
   conn->send("getData", message);