Review 'Markdown guide'

[jdbaldry]

• Add semantic line breaks
• Add some more meaningful alt text
• Make some clarifying edits

The rest LGTM

Closes https://github.com/grafana/writers-toolkit/issues/752

[github-actions[bot]]

Readability report

File	AutomatedReadability	ColemanLiau	FleschKincaid	FleschReadingEase	GunningFog	LIX	SMOG
docs/sources/write/markdown-guide/index.md	$${6.72\space\color{green}(Δ-0.06)}$$	$${10.33\space\color{green}(Δ-0.04)}$$	$${7.24\space\color{green}(Δ-0.04)}$$	$${59.88\space\color{red}(Δ-0.01)}$$	$${8.67\space\color{green}(Δ-0.02)}$$	$${35.67\space\color{green}(Δ-0.18)}$$	$${10.10\space\color{green}(Δ-0.03)}$$

View metric targets

| Metric | Range | Ideal score | | --- | --- | --- | | [AutomatedReadability](https://en.wikipedia.org/wiki/Automated_readability_index) | 6 (very easy read) to 14 (extremely difficult read) | 8 or less | | [ColemanLiau](https://en.wikipedia.org/wiki/Coleman%E2%80%93Liau_index) | 6 (very easy read) to 17 (extremely difficult read) | 9 or less | | [FleschKincaid](https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests) | 6 (very easy read) to 17 (extremely difficult read) | 8 or less | | [FleschReadingEase](https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests) | 100 (very easy read) to 0 (extremely difficult read) | 70 or more | | [GunningFog](https://en.wikipedia.org/wiki/Gunning_fog_index) | 6 (very easy read) to 17 (extremely difficult read) | 10 or less | | [LIX](https://en.wikipedia.org/wiki/Lix_(readability_test)) | 20 (very easy read) to 60+ (extremely difficult read) | 35 or less | | [SMOG](https://en.wikipedia.org/wiki/SMOG) | 6 (very easy read) to 17 (extremely difficult read) | 10 or less |