update CONTRIBUTING.md and github ISSUE_TEMPLATE (#30862)

Automatic Merge

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -16,7 +16,7 @@ body:
         required: true
       - label: This issue reproduces on one of the [currently supported server versions](https://docs.mattermost.com/about/mattermost-server-releases.html#latest-releases).
         required: true
-      - label: I have read the [contribution guidelines](https://github.com/mattermost/mattermost/blob/master/CONTRIBUTING.md).
+      - label: I have read the [contribution guidelines](https://github.com/mattermost/mattermost/blob/master/CONTRIBUTING.md) and the [Mattermost Handbook Contribution Guidelines](https://handbook.mattermost.com/contributors/contributors/guidelines/contribution-guidelines).
         required: true
 - type: input
   attributes:

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/).