docs: document known issue with JsonConverter under Scala 2 and update site navigation

- Added documentation for the `JsonConverter` issue with `Any` on Scala 2.
- Introduced a "Known Issues" section in the site navigation.

Author: QuadStingray

.gitignore

@@ -26,3 +26,4 @@ node_modules
 /docs/.vitepress/dist/
 /docs/external/files
 /docs/.vitepress/cache
+.pnpm-store