Enable using `gh-pages` branch for displaying the spec

[mingyc]

I've created the gh-pages branch, but it looks like this repo is not enabled to use it for https://wicg.github.io/unload-beacon

Not quite sure about the process here, but given that we already have a spec file (although it's not up-to-date with the explainer), should we enable the static page?

I also have a PR ready to setup github workflow to generate the update from *.bs file.

[clelland]

Yes, I think we need someone with admin access to the repo to set that up.

[fergald]

@yoavweiss should have access to enable GH pages.

[yoavweiss]

Done!! :)