Case studies index page and link from homepage

[aliok]

"Fixes #issue-number" or "Add description of the problem this PR solves"

Problem

• The "Case Studies" text in the homepage at knative.dev is currently not clickable. You need to click one of the company logos to see the full list.

Proposed Changes

• Made the "Case Studies" text in the homepage at knative.dev clickable. It points to a new page that lists all case studies in a table
• Preview:
  • Check out the "Case Studies" header at https://deploy-preview-5869--knative.netlify.app/docs/
  • Check out the new page: https://deploy-preview-5869--knative.netlify.app/docs/about/case-studies/