Adding clarification to RHL3 docs + JSX highlight in code blocks

[tsaiDavid]

@gadicc @hedgerh @gaearon - Went ahead and added the changes I mentioned along with some other little touches.

• Clarification and showing the JSX syntax with the AppContainer component wrapping RootContainer - hopefully this is more beginner friendly and less confusing since v3 will require a bit more config.
• Switched from js to jsx for the code blocks, if I'm not mistaken it's GitHub markdown friendly 👍
• Provided super basic start for us to show usage within webpack loader (just to get the ball rolling)

[tsaiDavid]

Just want to point out I was also able to update the shell command example, prefixed it with the $ symbol that we typically see, indicating that it is a command to be used from one's terminal.

An example was also not in consistent ES6 as in our other examples, so I updated it.

[tsaiDavid]

@gadicc #257 #243 - apologies for all the comments, just want to make sure I'm referencing the issue correctly, please let me know if there's anything else I can do.

[gadicc]

this is awesome, @tsaiDavid, thanks! all very welcome improvements. just one issue which I'll comment on in the diff and then I'm happy to merge. yeah, for future it's enough to just put a #257 in there somewhere and we'll get a notification (I think :)).

[gadicc]

just this snuck through :)

[gadicc]

@tsaiDavid I'm going to fix that line on my side, these were actually intended for the gh-pages (sorry that's my bad) so need to move these there and then I'll ping you again. Thanks!