Lint Markdown guide

[jdbaldry]

This also removes the guidance to avoid "single-child headings" because I can't find any justification for it online.

Closes #196

[github-actions]

Overall readability score: 60.61 (🔴 -0.03)

File	Readability
index.md	66.72 (🔴 -1.46)

View detailed metrics

🟢 - Shows an increase in readability
🔴 - Shows a decrease in readability

File	Readability	FRE	GF	ARI	CLI	DCRS
index.md	66.72	50.94	7.76	10.8	12.34	6.84
	🔴 -1.46	🔴 -0.91	🔴 -0.21	🔴 -0.3	🔴 -0.41	🟢 +0.02

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	60.61	47.42	9.53	11.68	12.15	7.3
	🔴 -0.03	🔴 -0.02	🟢 +0	🔴 -0.01	🔴 -0.01	🟢 +0

View metric targets

Metric	Range	Ideal score
Flesch Reading Ease	100 (very easy read) to 0 (extremely difficult read)	60
Gunning Fog	6 (very easy read) to 17 (extremely difficult read)	8 or less
Auto. Read. Index	6 (very easy read) to 14 (extremely difficult read)	8 or less
Coleman Liau Index	6 (very easy read) to 17 (extremely difficult read)	8 or less
Dale-Chall Readability	4.9 (very easy read) to 9.9 (extremely difficult read)	6.9 or less

[chri2547]

LGTM! Thank you for the contribution!