updated outdated loader explanation

[elenamik]

Changes proposed in this Pull Request

• As suggested by @jsnajdr in replacing outdated link #38271, webpack no longer uses require.ensure() statements. Instead, it adds a dynamic import() function to the load object.

Testing instructions

• Inspecting sections.js after webpack calls the loader, you can see that it adds a function, stored in section.load.
• It seems the loader was simplified significantly in Framework: introduce sections middleware #21349, but the documentation did not reflect the changes.

Fixes #

[elenamik]

@jsnajdr please review!
The one thing I am not sure about is how / if the page.js route handlers are generated now. It seems to work quite differently than what is written in the doc.

[lancewillett]

Re-ping @jsnajdr

[github-actions]

This PR has been marked as stale due to lack of activity within the last 30 days.