feat(playground): Use relative paths for assets, playground requests and fonts

[dacosta444]

Based on the work in PR #7816

"When attempting to run Cube behind a reverse proxy, it is only possible to set the base path for the API endpoints of Cube. This renders the playground unusable inside a non root base path.

For example when using /cube as a base path behind a reverse proxy, I can get the index.html to load, from /cube/index.html but then requests for assets are made to /some-asset.js instead of /cube/some-asset.js"

In addition to the changes made in that PR the requests to playground were still using the absolute path.
Fonts were also using an absolute path and the fix for that was forcing a relative path.

Check List

• Tests have been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue Reference this PR resolves
#3028 #7969

[jpsfs]

This is great work!
Hope this gets merged soon! 🙏

[dacosta444]

Would it be possible to trigger the action to build and publish a Docker image for this PR so I can use it while the review is still pending?

It would be super helpful to have a published image using the GitHub Actions flow — something similar to the build and publish step
cc @vasilev-alex, @ovr
Thanks a lot in advance!

[tenphi]

@dacosta444 Sorry for the long wait.