Refine the code examples section UI of index page

[gingerhot]

I hope I can help to refine the code examples section UI in the index page every time when I browse perl6.org. So I create this PR finally : )

Before:

After:

And I also set the <pre> tag a solid height to prevent a flashing when a tab is clicked. It's more than the height of the highest <pre> section at present. I guess the height is ok for such code snippets.

[gingerhot]

@moritz could you help me checking the SASS code? And someone here can help to review the PR? Thanks in advance.

[coke]

Unfortunately this has bitrotted and now conflicts with scss file; If you can re-generate a clean version of this, please open a new PR and ask for my review. Thanks for your efforts!