Add PWA manifest.json

What does this MR do and why?

This adds a manifest.json as a route and then links to it through the meta tag. This makes GitLab a fully supported PWA which then gives us as an next step the possibility to add a service_worker

Manifest is being loaded in the Meta tags on - https://gitlab-review-tz-add-pwa-w2m3be.gitlab-review.app/explore The manifest is located under - https://gitlab-review-tz-add-pwa-w2m3be.gitlab-review.app/-/manifest.json

How to set up and validate locally

1. Start GDK
2. See manifest.json loaded on every page through META tags

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.