[Elixir] 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

[angelikatyborska]

And now we can migrate exercise introductions to include concept introductions, to remove the duplication? Via https://github.com/exercism/v3-docs/blob/master/anatomy/tracks/concept-exercises.md#file-docsintroductionmdtpl-optional ?

It is the track's responsibility to generate the introduction.md when a template file is used.

I don't know what that means. What do we need to do extra?

[ErikSchierboom]

And now we can migrate exercise introductions to include concept introductions, to remove the duplication? Via https://github.com/exercism/v3-docs/blob/master/anatomy/tracks/concept-exercises.md#file-docsintroductionmdtpl-optional ?

You can, but there is one "but" (see response below)

It is the track's responsibility to generate the introduction.md when a template file is used.

I don't know what that means. What do we need to do extra?

Tracks will have to use configlet to generate the introduction.md from a template: https://github.com/exercism/v3-docs/blob/master/anatomy/tracks/configlet/generating-documents.md#document-concept-exercises-introductionmd-file. The Exercism website does not know about the template file and will only look at the introduction.md file. Tracks thus have to run configlet themselves in order to update introduction.md files.

At the moment though, configlet v3 doesn't yet support this generating. We hope to implement this soon.