Support for <indexterm>s when reading DocBook

[bigsmoke]

This PR resolves #7427. This was my first Haskell patch ever, so I'm sure that my additions, although functional, are an eyesore to seasoned Haskell coders. Please feel even more free than you usually would to correct/criticize my code.

[jgm]

Here it's idiomatic to omit the parentheses around (strContent naryEl).

[jgm]

Should we include the pagenum attribute?

What about see and seealso elements (there can be multiple of the latter)?

[jgm]

A test would be nice, too!
Easiest would be a "command test", since there isn't currently a Tests.Reader.DocBook module.

[bigsmoke]

Should we include the pagenum attribute?

I don't think so. That is only used within <index> sections, which, like we discussed in #7608, makes most sense to continue to ignore in Pandoc.

What about see and seealso elements (there can be multiple of the latter)?

I will follow up with a new PR request with (partial) support for <see> and <seealso> elements. They will be more straight-forward and less flaky to express as attributes to the span than <primar>, <secondary> and <tertiary>, because the latter actually have a mixed content model (which AsciiDoc doesn't have, so that it doesn't become a problem for me personally)..

A test would be nice, too!
Easiest would be a "command test", since there isn't currently a Tests.Reader.DocBook module.

I will included a test too. Thank you for the pointer! ☺

[bigsmoke]

I have improved the patch. You stimulated me to go a bit further and also do the Right Thing™ with mixed-content child element of <indexterm>, including a test case to ensure that the existing strContentRecursive that I reused indeed did what I hoped it would: to flatten the mixed content of, for example, a <primary> element into a string suitable for use in an attribute.

Thanks for sticking with me, @jgm ☺

[bigsmoke]

I reworded one of my commit messages and did a git push --force-with-lease, because one of the GitHub sanity check choked on its length 5 days ago.

[jgm]

Great, thanks.