Add F5 case study

Created by: aileenrose

https://github.com/sourcegraph/about/issues/1714

DRAFT Pull Request - Awaiting headshot for Satish Surapaneni from F5, if available.

For review: https://deploy-preview-1734--sourcegraph.netlify.app/case-studies/ https://deploy-preview-1734--sourcegraph.netlify.app/case-studies/f5-streamlines-collaboration-globally/