docs: add Jinjava 3.0 migration guide for open-source users

Jinjava 3.0 changes defaults, introduces a deny-by-default method/return-type
allowlist, and aligns more closely with Python/Jinja2 semantics. There was no
documentation helping external users navigate this upgrade — this guide fills
that gap with breaking change details, allowlist setup walkthroughs, migration
recipes, and reference tables.

Also updates README to link to the guide and corrects the Java version
requirement from 8 to 17.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jasmith-hs

README.md

@@ -9,7 +9,9 @@
 
 Java-based template engine based on django template syntax, adapted to render jinja templates (at least the subset of jinja in use in HubSpot content). Currently used in production to render thousands of websites with hundreds of millions of page views per month on the [HubSpot CMS](http://www.hubspot.com/products/sites).
 
-*Note*: Requires Java >= 8. Originally forked from [jangod](https://code.google.com/p/jangod/).
+**Upgrading from 2.x?** See the [Migration Guide](docs/migration-2.x-to-3.0.md).
+
+*Note*: Requires Java >= 17. Originally forked from [jangod](https://code.google.com/p/jangod/).
 
 Get it:
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