Javascript Get Started with code walkthrough

[maria-robobug]

Preview link here

[mDuo13]

This looks pretty good, but I do see some room to polish it up.

1. The CSS could be better. We should save this for a follow-up task, but just making note of what I see:
   • The font color of the code sample's non-focused areas feels a bit too low-contrast to me, especially since the entire code sample is "greyed out" much of the time. Some of the UI elements like the Environment selector are also low visibility in light mode.
   • The columns feel cramped to me and a little uneven too. Especially on my 1440p desktop monitor, there's wasted space due to the max width (1504px) of .code-walkthrough elements... I don't know why that's there, but removing it makes the fit a lot better on big monitors.
   • The margins between elements and the space for line numbers in the code sample feel a bit too small.
2. We should add a README to the js/ folder that sends people to the page where they can download the clean example. We might also want to provide a separate, clean example file in the repository, without the @chunk comments. That would involve maintenance to keep them from diverging, though, so maybe not.
3. It seems wrong to have this buried under "Build Applications..." That's something we can and should address later when we move the tutorials around as planned. After we do that, we should also update the _code-samples/get-started/README.md file to have the new links to the tutorials.

Otherwise, I noted a few places where the old format or language needs to be updated just a little. This should be good to go with those changes.

[maria-robobug]

Holding off on merging this, as per @amarantha-k's suggestion to wait for feedback from the design team.