Improve simulations design

[ryaanahmed]

@equationcrunchor points out in #438:

I think it looks good enough for merge, just some possible future improvements:

• The code in the demos unnecessarily scrolls horizontally, because Bootstrap can't seem to size it correctly if its scrollable
• The headings for the code table in the demos probably shouldn't be part of the scrollable section
• Padding is weird, especially at the top of words
• I don't like how the scroll bar doesn't show up unless you start scrolling, because in some demos you might not even notice that there's more content.

[ryaanahmed]

@srisi:

I'd make the #line_desc element in the simulations always at least 2 lines tall. As it stands, the boxes below (Help and Other Registers) constantly move up and down depending on the length of the line description.