Lint for abstract first because it breaks our schema markup

[hexylena]

e.g. https://training.galaxyproject.org/training-material/topics/single-cell/tutorials/GO-enrichment/tutorial.html doesn't get indexed by tess because it doesn't follow the GTN formatting guidelines of 'agenda after intro' and 'no massive intro header'

[hexylena]

maybe we need a more explicit style guide (#3935) so reviewers know not to merge things with agenda first or # Introduction first, so it can match the rest of the GTN's style.

[hexylena]

Also maybe lint for #5379