Add `review_date` front matter documentation

[github-actions[bot]]

Readability report

File	AutomatedReadability	ColemanLiau	FleschKincaid	FleschReadingEase	GunningFog	LIX	SMOG
docs/sources/write/front-matter/index.md	$${6.92\space\color{green}(Δ-0.07)}$$	$${9.36\space\color{green}(Δ-0.11)}$$	$${8.05\space\color{green}(Δ-0.10)}$$	$${59.81\space\color{green}(Δ+0.71)}$$	$${10.15\space\color{green}(Δ-0.11)}$$	$${37.03\space\color{green}(Δ-0.20)}$$	$${11.14\space\color{green}(Δ-0.08)}$$

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 |