Continuation of Acknowledgements page (from #10855) (#10873)

* [Gatsby docs update] WIP add Acknowledgements page

**what is the change?:**
Add custom template for 'acknowledgements' page

**why make this change?:**
We want this page to show up on the new docs site.

**test plan:**
Manual testing, BUT it doesn't work yet because the markdown file has
some logic that isn't supported by the plugins we are using.

See screenshot: (Flarnie will insert)

**issue:**
checklist item on Gatsby checklist
https://github.com/bvaughn/react/wiki/Gatsby-check-list

* Migrated acknowledgements from template to page