Update README

[ghost]

Some remarks:

• GitHub-markdown changed, or some code-blocks in the README were always broken. Syntax-highlighting was added (only some blocks had it).
• Replaced URLs with new equivalents. For gh-pages, GitHub prefers github.io now. This also affects the homepage URL in the project sidebar, which should be updated by @benvanik.
  I believe this might actually fix Directly use embed.js failed, try loading core/lib/gli.js, but gli.js isn't there? #167 .
  (I had the same issue a couple of days ago, but don't remember how I resolved it - I thought it was the ".com → .io"-change, but wasn't able to reproduce the issue anymore while writing this PR).
• I'm not sure if there's a replacement for the Firefox extension or if it even still works when built manually. I removed the dead link to pre-build but kept the rest, assuming it still works.
• I kept the http://learningwebgl.com/ link even though it is currently unavailable. It has been working until some months ago, so chances are it will return.

There are other minor things in the README I'd change, but I'm not even sure if this repository is still maintained.
If it isn't, we should update the README to refer people to alternative tools with equivalent functionality, then archive the repository.
If it is, I'll probably send some follow-ups in the future (documentation and code alike, if there's need).

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(It would be nice if you can tag this PR for hacktoberfest inclusion as described in https://hacktoberfest.digitalocean.com/details#rules - feel free to ignore this last bit if you don't want this)