[Super Cache] Add docs explaining preload constants to Preload page

Create an "Advanced" section on the preload page with docs explaining the new constants introduced in Automattic/jetpack#36246.
* WPSC_PRELOAD_POST_INTERVAL is the interval between preloaded posts when they are fetched. (default: 1 second)
* WPSC_PRELOAD_LOOP_INTERVAL is the interval between each loop. (default 0 seconds, plus 3 hard coded scheduled cron job)

There is also the WPSC_PRELOAD_POST_COUNT constant that sets the number of preloaded posts. (default 10)

Start from the existing Super Cache Preload page and review its current structure before adding an Advanced section. Document WPSC_PRELOAD_POST_INTERVAL, WPSC_PRELOAD_LOOP_INTERVAL, and WPSC_PRELOAD_POST_COUNT with their stated defaults and meanings. Done means the three constants and their behavior are clearly explained on that page.

Written by the indexing model from the issue text.