Fix bugs in guide-css page
Add basic CSS for lessons

This is a mix of old and new, with an eye toward making the style rules
cleaner and easier to understand.
Add lesson: voiced-and-unvoiced-sounds
Add lesson: aspirated-consonants
Add lesson: s-consonants
Add lesson: ha
Add lesson: anusvara-and-visarga
Add lesson: the-sanskrit-alphabet
Add lesson: special combinations
Change 'row_ul' macro to accept callbacks
Add lesson: how-sanskrit-works
Add lesson: syllables
Change .rule CSS
Remove sentence fragment in simple-vowels

How did that get there?
Add lesson: devanagari
Add lesson: marks
Add lesson: numerals
Add lesson: punctuation
Add lesson: devanagari-consonants
Add lesson: consonant-clusters