update CONTRIBUTING.md and github ISSUE_TEMPLATE (#30862)

Automatic Merge

CONTRIBUTING.md

@@ -2,4 +2,6 @@
 
 Thank you for your interest in contributing! Please see the [Mattermost Contribution Guide](https://developers.mattermost.com/contribute/getting-started/) which describes the process for making code contributions across Mattermost projects and [join our "Contributors" community channel](https://community.mattermost.com/core/channels/tickets) to ask questions from community members and the Mattermost core team.
 
+In addition, we recommend reviewing the [Contribution Guidelines](https://handbook.mattermost.com/contributors/contributors/guidelines/contribution-guidelines) in the Mattermost Handbook, which provide comprehensive best practices and expectations for contributors.
+
 When you submit a pull request, it goes through a [code review process outlined here](https://developers.mattermost.com/contribute/getting-started/code-review/).