Add web app manifest for PWA homescreen install

Add web app manifest for PWA homescreen install

Part of gitlab-com/www-gitlab-com#7652 (closed) which is a part of our bigger PWA work: https://gitlab.com/gitlab-org/gitter/webapp/-/issues/1992

Chrome devtools

Chrome devtools -> Application -> Manifest

Installation

No matching service worker detected. You may need to reload the page, or check that the scope of the service worker for the current page enclosed the scope and start URL from the manifest.

Dev notes

• https://developer.mozilla.org/en-US/docs/Web/Manifest
• https://web.dev/add-manifest/