Docs - Update link in SSL_TLS Certificates Page to Let's Encrypt Intergration

What does this MR do?

The docs page SSL/TLS Certificates has a link to a deprecated page Let's Encrypt for GitLab Pages. I updated the link description and URL to the page GitLab Pages integration with Let's Encrypt.

Does this MR meet the acceptance criteria?

I hope so; this is my first time using Git and GitLab. Approver: Is the language I used for the link acceptable?

Performance and testing

• Does the link work after updating?

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.