Update markdown lint issue with headings

[chartgerink]

This PR follows up the discussion in #76.

The remaining lint issues are line length (not a big deal here) and alt text for the images (which would be preferable for accessibility purposes).

Remaining lint issues

vignettes/cohort_design.Rmd:25:81 MD013/line-length Line length [Expected: 80; Actual: 1032]
vignettes/cohort_design.Rmd:31:81 MD013/line-length Line length [Expected: 80; Actual: 837]
vignettes/cohort_design.Rmd:33:81 MD013/line-length Line length [Expected: 80; Actual: 120]
vignettes/cohort_design.Rmd:35:81 MD013/line-length Line length [Expected: 80; Actual: 485]
vignettes/cohort_design.Rmd:37:81 MD013/line-length Line length [Expected: 80; Actual: 553]
vignettes/cohort_design.Rmd:39:81 MD013/line-length Line length [Expected: 80; Actual: 150]
vignettes/cohort_design.Rmd:41:81 MD013/line-length Line length [Expected: 80; Actual: 211]
vignettes/cohort_design.Rmd:41:1 MD045/no-alt-text Images should have alternate text (alt text)
vignettes/cohort_design.Rmd:45:81 MD013/line-length Line length [Expected: 80; Actual: 1034]
vignettes/cohort_design.Rmd:47:81 MD013/line-length Line length [Expected: 80; Actual: 113]
vignettes/cohort_design.Rmd:79:81 MD013/line-length Line length [Expected: 80; Actual: 327]
vignettes/cohort_design.Rmd:83:81 MD013/line-length Line length [Expected: 80; Actual: 584]
vignettes/cohort_design.Rmd:85:81 MD013/line-length Line length [Expected: 80; Actual: 212]
vignettes/cohort_design.Rmd:85:1 MD045/no-alt-text Images should have alternate text (alt text)
vignettes/cohort_design.Rmd:87:81 MD013/line-length Line length [Expected: 80; Actual: 154]
vignettes/cohort_design.Rmd:91:81 MD013/line-length Line length [Expected: 80; Actual: 703]
vignettes/cohort_design.Rmd:93:81 MD013/line-length Line length [Expected: 80; Actual: 211]
vignettes/cohort_design.Rmd:93:1 MD045/no-alt-text Images should have alternate text (alt text)
vignettes/cohort_design.Rmd:95:81 MD013/line-length Line length [Expected: 80; Actual: 721]
vignettes/cohort_design.Rmd:97:81 MD013/line-length Line length [Expected: 80; Actual: 505]
vignettes/cohort_design.Rmd:99:81 MD013/line-length Line length [Expected: 80; Actual: 95]
vignettes/cohort_design.Rmd:121:81 MD013/line-length Line length [Expected: 80; Actual: 95]
vignettes/cohort_design.Rmd:127:81 MD013/line-length Line length [Expected: 80; Actual: 662]
vignettes/cohort_design.Rmd:128:81 MD013/line-length Line length [Expected: 80; Actual: 100]
vignettes/cohort_design.Rmd:144:81 MD013/line-length Line length [Expected: 80; Actual: 306]
vignettes/cohort_design.Rmd:146:81 MD013/line-length Line length [Expected: 80; Actual: 155]
vignettes/other_designs.Rmd:21:81 MD013/line-length Line length [Expected: 80; Actual: 181]
vignettes/other_designs.Rmd:27:81 MD013/line-length Line length [Expected: 80; Actual: 141]
vignettes/other_designs.Rmd:29:81 MD013/line-length Line length [Expected: 80; Actual: 91]
vignettes/other_designs.Rmd:31:81 MD013/line-length Line length [Expected: 80; Actual: 250]
vignettes/other_designs.Rmd:37:81 MD013/line-length Line length [Expected: 80; Actual: 153]
vignettes/other_designs.Rmd:39:81 MD013/line-length Line length [Expected: 80; Actual: 151]
vignettes/other_designs.Rmd:41:81 MD013/line-length Line length [Expected: 80; Actual: 171]
vignettes/other_designs.Rmd:43:81 MD013/line-length Line length [Expected: 80; Actual: 185]
vignettes/other_designs.Rmd:47:81 MD013/line-length Line length [Expected: 80; Actual: 272]
vignettes/other_designs.Rmd:49:81 MD013/line-length Line length [Expected: 80; Actual: 202]
vignettes/other_designs.Rmd:51:81 MD013/line-length Line length [Expected: 80; Actual: 185]
vignettes/references.bib:1 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "@article{habibzadeh2022measuri..."]
vignettes/references.bib:29:81 MD013/line-length Line length [Expected: 80; Actual: 164]
vignettes/vaccineff.Rmd:25:81 MD013/line-length Line length [Expected: 80; Actual: 490]
vignettes/vaccineff.Rmd:27:81 MD013/line-length Line length [Expected: 80; Actual: 318]
vignettes/vaccineff.Rmd:31:81 MD013/line-length Line length [Expected: 80; Actual: 405]
vignettes/vaccineff.Rmd:35:81 MD013/line-length Line length [Expected: 80; Actual: 546]
vignettes/vaccineff.Rmd:39:81 MD013/line-length Line length [Expected: 80; Actual: 467]
vignettes/vaccineff.Rmd:43:81 MD013/line-length Line length [Expected: 80; Actual: 119]
vignettes/vaccineff.Rmd:47:81 MD013/line-length Line length [Expected: 80; Actual: 556]
vignettes/vaccineff.Rmd:51:81 MD013/line-length Line length [Expected: 80; Actual: 92]
vignettes/vaccineff.Rmd:55:81 MD013/line-length Line length [Expected: 80; Actual: 164]
vignettes/vaccineff.Rmd:57:81 MD013/line-length Line length [Expected: 80; Actual: 206]
vignettes/vaccineff.Rmd:59:81 MD013/line-length Line length [Expected: 80; Actual: 94]
vignettes/vaccineff.Rmd:61:81 MD013/line-length Line length [Expected: 80; Actual: 133]
vignettes/vaccineff.Rmd:63:81 MD013/line-length Line length [Expected: 80; Actual: 175]
vignettes/vaccineff.Rmd:65:81 MD013/line-length Line length [Expected: 80; Actual: 306]
vignettes/vaccineff.Rmd:77:81 MD013/line-length Line length [Expected: 80; Actual: 281]
vignettes/vaccineff.Rmd:79:81 MD013/line-length Line length [Expected: 80; Actual: 136]
vignettes/vaccineff.Rmd:83:81 MD013/line-length Line length [Expected: 80; Actual: 314]
vignettes/vaccineff.Rmd:84:81 MD013/line-length Line length [Expected: 80; Actual: 180]
vignettes/vaccineff.Rmd:86:81 MD013/line-length Line length [Expected: 80; Actual: 198]
vignettes/vaccineff.Rmd:88:81 MD013/line-length Line length [Expected: 80; Actual: 222]
vignettes/vaccineff.Rmd:122:81 MD013/line-length Line length [Expected: 80; Actual: 189]