Replace "/users" page with case studies

I think a [simple list of companies](https://graphql.org/users) was useful in the early days but today GraphQL usage is so widespread (especially internally) that it doesn't add too much value anymore.
I suggest replacing it with a case study page similar to the one that Apollo has:
https://www.graphql.com/case-studies/

With simple rules:
* Article should be written on the official blog page
* It should describe the adoption of GraphQL as an end-user (a meaning company is hosting GraphQL API), not a tool/library.
* It should have more content than a simple announcement (e.g. why the decision was made, migration strategy, pros and cons, etc.)
* Submissions are sorted alphabetically based on company name.

Review the existing /users page on graphql.org and compare it with Apollo's case-studies page linked in the issue. Define the page structure and submission criteria around substantial end-user adoption stories, then confirm that accepted entries are sorted alphabetically by company name.

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