Add docs for configurable event subscribers

[benjaminwil]

Summary

Built-in Solidus event subscribers are now configurable since solidusio/solidus#6234 was merged. The docs added by this pull request help users understand how to add or remove built-in event subscribers in their applications.

Checklist

• I have followed the Diátaxis framework in my PR.
• I have verified that the preview environment works correctly.

[benjaminwil]

The preview environment has not yet been generated, so I can't complete the checklist yet.

[tvdeyen]

thanks

[benjaminwil]

The expected Cloudflare Pages check does not seem to be running against any of the currently open pull requests.

It is unclear whether this is because a core committer needs to allow the check to run from an untrusted collaborator or whether something is broken.

[benjaminwil]

The Cloudflare Pages preview hasn't built as far as I can tell. But I manually verified that the documentation is rendering well locally.

[benjaminwil]

CCing people from the core team who might know. @tvdeyen @kennyadsl @jarednorman

I've grepped this repository for "cloudflare" and haven't found any information about how to make the Cloudflare Pages previews work. Setting that aside, I think this pull request is mergable.