Carpentries review comments

[alanocallaghan]

Per review in: carpentries-lab/reviews#19

• Fix heading levels (remove h1s)
• Improve table alt-text (ch1), pairsplot alt-text (ch4), heatmap alt-text (ch 7).
• Add missing table header (ch2)
• Remove slides, as these are likely to suffer substantial drift from lesson content and shouldn't be necessary :)
• Add comment in instructor notes about managing cognitive load.

[catavallejos]

Thanks for making all these changes @alanocallaghan . Looks good! And I agree that having the slides is redundant and risks having them non up-to-date.

[alanocallaghan]

Thanks! Will blitz the paper #175 early next year inshallah