dnd-character: split into instructions and introduction

[ErikSchierboom]

• dnd-character: convert Note heading to exercism/note
• dnd-character: grammar fix
• dnd-character: split up into introduction and instructions

[BethanyG]

Couldn't make a suggestion, since this is a part of the exercise that didn't change. On line 6 of the instructions.md file:

You do this by rolling four 6-sided dice and record the sum of the largest three dice.

Should either be:

You do this by rolling four 6-sided dice and recording the sum of the largest three dice. or These six abilities have scores that are determined randomly by rolling four 6-sided dice. The ability score should be recorded as the sum of the largest three dice.

[ErikSchierboom]

@BethanyG Updated!