Create and publish "release" branch to GitHub Pages

[tidoust]

Currently, the master branch is published to GitHub Pages. That means the index.json file may include changes that have not yet been integrated in the NPM package and in the GitHub release. For improved consistency, it would be good to rather maintain a release branch that always points at the latest release, and to publish that branch instead.

[foolip]

If releases are created by tagging the repository, it would be fairly straightforward to automatically roll a release branch forward using a GitHub Action.

[tidoust]

With the release workflow introduced while adding support for non-browser specs (PR #496, issue #436), the web-specs@latest branch is now the one published under GitHub Pages. It should always point to the commit that gave birth to the latest released version of the web-specs package. In other words, the web-specs@latest branch acts as the release branch envisioned by this old issue.

FWIW, the index.json file on the main branch is now only updated when tests pass (it used to be updated no matter what).