This is a 🐛 bug fix.
I have not run the CI suite.

Summary

When running with bundle exec jekyll serve and no other options, the absolute_url filter will now use the local server URL instead of the URL from the config (like benlk.com) when that URL is the default localhost:4000.

Context

This PR is designed to fix the regression introduced in #7253 and described in my comments there: #7253 (comment)

If you don't want to check the links, here's a summary:

• Don't reset site.url to localhost:4000 by default #7253 changed the behavior of jekyll serve to use the site URL from the site's config when serving a local development version of the site, if the localhost host and port were not explicitly specified. This meant that site.url and the absolute_url filter would use the production URL when generating and serving the site locally. The documentation on jekyllrb.org was not updated to reflect this change in behavior, so people who innocently ran jekyll serve and expected assets to load from their local development server would instead see:
  • images loaded from the production server, or 404 if not present on the production server
  • stylesheets loaded from the production server without changes from edits made locally, or 404 if not present on the production server
  • scripts loaded from the production server without changes from edits made locally, or 404 if not present on the production server
  • font files failing to load from production servers not configured to allow CORS requests for font files from localhost:4000, or 404 if not present on the production server
• That change did not affect cases where the local server URL was explicitly set via bundle exec jekyll serve --host localhost --port 4000 to be localhost:4000, or when it was set to be any other combination of hosts or ports. That change only affected the default jekyll serve, which is the local development action recommended in the Jekyll docs.

This PR does update documentation with the recommendations from discussion in #7253

Reverts #7253