[Common Lisp] 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

[verdammelt]

Hmm when I did the stubbing I had 36 files - but there are only 16 files here. I guess this is only stubbing out concepts mentioned in current exercises. (This track has concepts listed in the config.json (and the related concept directories & files) that are not yet in any exercise.

[ErikSchierboom]

Hmmm. Let me check

[ErikSchierboom]

/format

[ErikSchierboom]

@verdammelt Found and fixed the bug!

[ErikSchierboom]

/format