Add Concept Exercise Metadata files: the-realm-of-echoes

[0xNeshi]

Towards #103

Scaffold https://github.com/exercism/cairo/blob/main/exercises/concept/the-realm-of-echoes

TODO:

Use other implemented concept exercise metadata files for inspiration, e.g. https://github.com/exercism/cairo/tree/main/exercises/concept/lucians-luscious-lasagna.

• instructions.md already implemented
• implement introduction.md
• implement hints.md
• implement ./meta/design.md
• implement the example solution in .meta/exemplar.cairo
• exercise stub at src/lib.cairo already implemented
• tests for the exercise at tests/the_realm_of_echoes.cairo already implemented
• replace <your_gh_username> placeholder with your actual GH username in .meta/config.json
• add missing blurb to .meta/config.json