Update CONTRIBUTING.md to link to issue templates (#1148)

Point to the new issue-creation page, now that we have templates for new issues.

CONTRIBUTING.md

@@ -4,5 +4,5 @@ You are eager to contribute to Grist? That's awesome! See below some contributio
 - [translate](/documentation/translations.md)
 - [write tutorials and user documentation](https://github.com/gristlabs/grist-help?tab=readme-ov-file#grist-help-center)
 - [develop](/documentation/develop.md)
-- [report issues or suggest enhancement](https://github.com/gristlabs/grist-core/issues/new)
+- [report issues or suggest enhancement](https://github.com/gristlabs/grist-core/issues/new/choose)