1. Add examples for the common string functions (upper, lower, and reverse)
2. Rename columns in datetime examples to be more informative (e.g. from 'd' to 'date')
3. Add examples for unix_timestamp, from_unixtime, rand, randn, collect_list, collect_set, lit,
4. Add note to all trigonometry functions that units are radians.
5. Add links between functions, (e.g. add link to radians from toRadians)