[Lasagna] revision to existing concept exercise

[colinleach]

For discussion on the forum

This revision (the most recent of many):

• Rewrites and shortens the introduction.md to meet Exercism guidelines.
• Enforces use of a constant, in place of magic numbers within the code.
• Points the top-level config.sys to the basics concept, instead of 3 older ones.

The old documentation isn't wasted: I moved some to basics/about.md and some to later concepts.

Including the introduction.md.tpl was my mistake. It should probably be removed now we no longer need to combine bits from 3 concepts.