CSS cleanup

Two changes in this patch:

**(1)** Two repeating CSS chunks were moved to their own files:

- Yahoo's "reset" styles were moved to '_reset_css.lua'.
- The code prettifier's styles were moved to '_code_css.lua'.

(The underscore at the filename's start is to show that it's not an
independent file but intended to be embedded in another.)

**CAVEAT: the two new files need to be added to the 'rockspec' file.**

**(2)** Everything in "pre.example" that's already in "pre" was removed
(leaving just the font-size).

Trivia facts:
- Just one copy of the "reset" style contained the correct "list-type: disc". The others contained an incorrect "list-type: bullet".
- ldoc_fixed_css.lua and ldoc_pale_css.lua mixed up "silver" with "#cccccc" (in pre.example). I had to decide on one, so I picked "silver" (translating it to "#C0C0C0").