Simplify workflows once GitHub releases PR previews

[vincerubinetti]

GitHub is working on built-in support for deploy previews on PRs:
community/community#7730

It seems like the feature is in alpha right now, and my guess is it will be ready sometime this year (hopefully very soon!).

Once this becomes available, it will drastically simplify the template template:

• A gh-pages branch will no longer be needed, and thus we can get rid of that step in the first time setup workflow.
• rossjrw/pr-preview-action will no longer be needed.
• The complicated separation and chaining of workflows can go away. Currently needed because a commit from one workflow cannot automatically trigger another workflow, but this will not apply to the built-in triggering of a Pages build. I expect to be able to cut down the number of files and lines of code in .github/workflows by half or more.

[vincerubinetti]

GitHub just added this nice little feature:

So I can get rid of the "update readme" step in the update-url workflow. Currently, it's still needed, because it is a (bad) way to detect if the Pages url has changed and whether to rebuild the site with a new baseurl. But once this issue is implemented, we can add instructions to check this checkbox, and update-url workflow won't be needed at all.