[v3] Update Markdown files to latest spec

[ErikSchierboom]

We've defined a specification for Markdown files to be applied Exercism-wide. This standard includes, amongst others, the following two rules:

• All files must start start with a level-1 heading (# Some heading text)
• No heading may decend a level greater than one below the previous (e.g. ## may only be followed by ###, not ####)

This PR applies the above two rules to the Markdown documents in this repo.

Tracking

https://github.com/exercism/v3-launch/issues/17