Install doc dependencies before building

[karkhaz]

This fixes a Graphviz graph not being rendered in the Kani book.

Description of changes:

Currently this page of the Kani book contains Graphviz source code. This is supposed to be rendered as an actual diagram, similar to this page (or this screenshot if that page disappears)

Resolved issues:

N/A

Related RFC:

N/A

Call-outs:

N/A

Testing:

• How is this change tested?

This successful run on my fork resulted in the diagram being rendered in my this page (screenshot above in case I push something else meanwhile)

• Is this a refactor change?

No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.