Add fingerprint mismatch documentation for SAML

What does this MR do and why?

This MR updates the Troubleshooting SAML docs to include information on how to calculate the SHA1 fingerprint.

This MR should help to make SAML setup a bit more straightforward and should help ease troubleshooting Fingerprint mismatch errors.

This was motivated by lessons learned while troubleshooting a ticket available to GitLab team members with access to Zendesk.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.