Slosh docstrings like (doc 'ns) in namespaces.md cause errors and prevent mdBooks from building

[StevenLove]

namespaces.md has five such errors preventing the script ./mk-site.sh from completing

[2024-09-21T00:28:13Z DEBUG mdbook_slosh_eval::slosh_eval_lib] Evaluate slosh code block #4 in chapter: Namespaces
[2024-09-21T00:28:13Z DEBUG mdbook_slosh_eval::slosh_eval_lib] New Code Block: (doc 'ns)
    
    ;; => Encountered error: [compile]: Symbol usage not defined (maybe you need to use 'def usage' to pre-declare it).

This one caused by the source code:

<!-- File: doc/src/namespaces.md -->
```slosh
  (doc 'ns)
```

After the 5 errors are reported, the thread panics:

thread 'main' panicked at slosh_test/src/docs.rs:677:16:
Building failed: Unable to parse the preprocessed book from "slosh-eval" processor

---

NOTE: (doc 'ns) works fine in ./target/debug/slosh. However, perhaps the preprocessor doesn't have access to the functions (like doc) defined in core.slosh.