Added a warning about removing old email addresses affecting commit associations.

Why:

Users may remove an old email address from their account without realizing that commits made with that email may no longer be associated with their profile. When this happens, those commits may stop appearing in the user's contribution graph.
This behavior is not currently mentioned in the documentation for changing the primary email address, which can lead to confusion or accidental loss of visible contribution history.

Closes: N/A

What's being changed (if available, include any code snippets, screenshots, or gifs):

This pull request adds a warning note before the step that instructs users to remove an old email address.
The warning explains that removing an email address used in previous commits may cause those commits to no longer be associated with the user’s account or appear in their contribution graph. It also advises users to ensure their commits are linked to another verified email before removing the address.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.