reset player snapshot after teleport to avoid deleting join location

[steve4744]

1. The player data snapshot is being reset before the teleportCourseCompletion() method is called. If the player should be teleported to their join location, this will no longer be recorded in the snapshot data. Fixed by moving the reset to after the teleport.

2. The %parkour_player_course_position_[course]% placeholder only works if there is a single time recorded in the database for the player on that course. If the player has multiple times the result is fairly random. Fixed by getting the best time rather than the first time.

3. The %parkour_current_checkpoint_next% code was missing, so this has been added.

4. The length check for %parkour_player_course_xxxxx% is now fixed.

5. Updated docs with an example of how to create Parkour holograms using the DecentHolograms plugin.