[Elixir] exceptions - replace about.md file with concept files

[ErikSchierboom]

In this issue we're describing an evolution of the specification of this repo.
These changes include, amongst others, replacing the contents of the after.md document with individual concept documents. There are two documents per concept:

1. An about.md file containing the description of the concept
2. A links.json file containing a set of useful links related to the concept.

This looks as follows in the repo:

<track>
├── concepts
│   ├── <concept>
│   │   ├── about.md
│   │   └── links.json
│   └── ...

This PR applies this change to this exercise by:

1. Copying the existing after.md file's contents to the about.md file of each concept listed in the exercise's concepts section in the config.json file.
2. Extract the links from the after.md file and put them into the links.json file

Before merging this PR, please check that the contents of the concept documents makes sense.
This is especially true when the exercise unlocks multiple concepts, as then the concept documents should be updated to only contain the information relevant to that concept.

Note: to make reviewing easier, the PR has been split into three separate commits. Please squash this PR upon merging.

[angelikatyborska]

No significant changes here.