PR summary

As suggested in #29699 , I updated the top message of the matplotlibrc file.

From: ## NOTE FOR END USERS: DO NOT EDIT THIS FILE!

To: ## DO NOT EDIT THIS FILE, MAKE A COPY FIRST

For me, this makes it more obvious that this file can be modified, as long as it is a copy of the original. The current message suggests that users should never touch this file. Even if the text just after says you can make a copy, I think a lot of people will stop reading because of the uppercase message at the top.

See the discussion here.

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines