[JavaScript] Add concept introductions

[ErikSchierboom]

We've recently specified that each concept should have its own introduction.md file: https://github.com/exercism/v3-docs/blob/master/anatomy/tracks/concepts.md#file-introductionmd.

This PR tries to automatically add these introduction.md files for each of the concept of your track. It does that as follows:

• If there is an exercise that teaches that concept:
  • If the exercise has a level 2 heading that matches the concept slug: create the introduction.md file with the contents directly following the level 2 header
  • Else: create the introduction.md file with its contents being the exercise's entire introduction.md file, but with a TODO added to the top
• Else: create the introduction.md file containing a TODO placeholder

See https://github.com/exercism/v3/issues/2975

[ErikSchierboom]

/format

[ErikSchierboom]

/format

[ErikSchierboom]

/format