Add alt text to images

[fstrr]

Eight images in WCAG 2.2 were missing alt attributes. This PR adds those in.

Ref: #1139

[scottaohara]

pending the remnant figcaption which is no longer needed, since the parent was changed to a div, this otherwise looks good!

[fstrr]

pending the remnant figcaption which is no longer needed, since the parent was changed to a div, this otherwise looks good!

Good catch. I thought I'd caught all of those. Thanks.

[mbgower]

I opened an html-aam issue on how figcaption should be able to serve as alt text a few years ago. IMO, this PR is a clear example of why that made sense. For many of the changes in this PR, the figcaption is being removed and instead its string is being used verbatim as the alt string.
Reason? Because despite the clear programmatic association between the image and figcaption, the accname doesn't recognize it as part of the formula.

Result: screen reader users receive the identical string of information that ALL users could have received. While I am in no way saying that the string should be unavailable to screen reader users, it's difficult to argue that depriving all sighted users of what was considered a useful string of information is an improved outcome.

I also think this re-use of the caption string as the ALT string serves as a perfect example of how the text of a figcaption and alt need not materially be different at all: #852 (comment)