Generate Diviner book for Javelin

Summary: And also fix a bug in the generator script.

Test Plan: Run bin/diviner generate, wait till the cows come home, see 3 pages about Javelin in the Diviner app.

Reviewers: O1 Blessed Committers, chris

Reviewed By: O1 Blessed Committers, chris

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25429