docs: explain the nullability spellings, and document the VER rules Two new pages and the reference entries behind them. Working with Nullability answers the question people actually ask when their models change - who decided this, and why. The OpenAPI Initiative removed `nullable` in February 2021; parsers honoured it for four more years; a 2025 release stopped. It carries the old and new spellings side by side, the rule that `nullable` only applies beside a `type`, and a support matrix for what this generator accepts at each version. Upgrading to v2 is the consumer-facing version: am I affected, how do I tell, what do I write instead. It moves here from `docs/` in the code repository, where a second copy would only drift. Analyzer Rules gains the VER category and a section per rule, each with the offending YAML and its rewrite. The rule headings are now bare ids so the help links in the diagnostics resolve to them - the CLT headings are trimmed for the same reason.