update pb-cgh-formulas-2

- add intro which was missing
- rename i to n, so the imaginary unit can be denoted i
- sync summation notation with the first part of this chapter
(see 3b55dcf23)
- add missing distance (radius) formula
- add <p> markup to fix rendering
add line breaks for readability & cleaner diffs
add some unfinished notes

attempting to highlight the time/space duality of the wave
and simplify the formulas
(or at least make them more intuitive)

also, remove 80% font-size,
as it becomes too small in large screens