URL management: redirect control panel + documentation

[nils-pzr]

Follow-up from @stevepiercy's review of #110

Already works

When content is renamed or moved, the Plone backend records an alias and Aurora already redirects the old URL to the new one automatically.

This is handled in the fetchPloneContent middleware, which forwards backend 3xx responses and flattens the location.

Verified end-to-end against a running backend.

Missing

1. Control panel UI for manual redirect management

A control panel UI is still missing to manage redirects manually, including:

• list site-wide aliases
• add site-wide aliases
• delete site-wide aliases

The URL Management entry in the control panels list already links to:

/controlpanel/aliases

However, no such panel exists yet.

Related notes:

• The icon is currently commented out in packages/cmsui/config/controlpanels.ts.
• @plone/client already ships the @aliases methods:
  • getAllAliases
  • createAliases
  • deleteAliases

2. Documentation of automatic redirect behavior

The automatic redirect behavior still needs to be documented.

docs/how-to-guides/manage-folder-contents-in-bulk.md currently contains a {todo} placeholder for #110.

That placeholder should be replaced with a reference once the behavior is documented.