mdbook-mermaid is missing from docs build+deploy workflow

[jeckersb]

Causing docs publishing to fail

Run cd docs && mdbook build
2024-05-06 17:38:41 [INFO] (mdbook::book): Book building has started
2024-05-06 17:38:41 [WARN] (mdbook::preprocess::cmd): The command wasn't found, is the "mermaid" preprocessor installed?
2024-05-06 17:38:41 [WARN] (mdbook::preprocess::cmd): 	Command: mdbook-mermaid
2024-05-06 17:38:41 [INFO] (mdbook::book): Running the html backend
Error: -06 17:38:41 [ERROR] (mdbook::utils): Error: Rendering failed
Error: -06 17:38:41 [ERROR] (mdbook::utils): 	Caused By: Unable to copy across additional CSS and JS
Error: -06 17:38:41 [ERROR] (mdbook::utils): 	Caused By: Unable to copy /home/runner/work/bootc/bootc/docs/mermaid.min.js to /home/runner/work/bootc/bootc/docs/book/mermaid.min.js
Error: -06 17:38:41 [ERROR] (mdbook::utils): 	Caused By: No such file or directory (os error 2)
Error: Process completed with exit code 101.

The mdbook-mermaid requirement was added in #503 but we forgot to add it to the docs workflow.