Use CSS to format coding exercises tables

[mferrare]

Table currently rendered as HTML inside a MD file. Need CSS to format properly.

[mferrare]

• Exercise number should be in bold
• Cell alignment should be top-left
• make the borders nicer
• banded rows would also be nice

[mferrare]

Oh, we get banding already.

Text could be a bit tighter. Too much spacing atm.