docs: Add issue scope to contribution guide (#8423)

Author: mtrezza

CONTRIBUTING.md

@@ -3,6 +3,7 @@
 ## Table of Contents <!-- omit in toc -->
 - [Contributing](#contributing)
   - [Issue vs. Pull Request](#issue-vs-pull-request)
+  - [Scope](#scope)
   - [Templates](#templates)
 - [Why Contributing?](#why-contributing)
 - [Contribution FAQs](#contribution-faqs)
@@ -65,6 +66,14 @@ An issue is required to be linked in every pull request. We understand that no-o
 - The discussion in the issue is different from the discussion in the pull request. The issue discussion is focused on the issue and how to address it, whereas the discussion in the pull request is focused on a specific implemention. An issue may even have multiple pull requests because either the issue requires multiple implementations or multiple pull requests are opened to compare and test different approaches to later decide for one.
 - High-level conceptual discussions about the issue should be still available, even if a pull request is closed because its appraoch was discarded. If these discussions are in the pull request instead, they can easily become fragmented over multiple pull requests and issues, which can make it very hard to make sense of all aspects of an issue.
 
+### Scope
+
+An issue and pull request must limit its scope on a distinct issue. Pull requests can only contain changes that are required to address the scoped issue. While it may seem quick and easy to add unrelated changes to the pull request, it can cause singificant complications after merging. Some of the reasons are:
+
+- A pull request corresponds to a single changelog entry. A changelog entry should not describe multiple unrelated changes in one entry for better readability.
+- A pull request creates a distinct commit; having an individual commit for each limited scope makes it easier for others to go back in the commit history and debug. Bugs are generally more difficult to identify and fix if there are various unrelated changes merged at once.
+- If a pull request needs to be reverted, unrelated changes will be reverted as well. That makes it more complex and time consuming to revert, having to consider its effects and possibly publishing a broken release or requiring a follow-up pull request with code manipulation.
+
 ### Templates
 
 You are required to use and completely fill out the templates for new issues and pull requests. We understand that no-one enjoys filling out forms, but here is why this is beneficial for everyone: