Document the procedure to migrate to scopes

Migrating Debusine to scoped workspaces happens automatically in data migrations by assigning all unscoped resources to the debusine scope, and having the DEBUSINE_DEFAULT_SCOPE setting default to debusine.

It is however unlikely that a site admin will want "debusine" as the scope name for their work, and we need to document a procedure to transition from there to what one wants.

This is likely to these few steps:

• decide a scope name
• stop debusine
• rename the "debusine" scope to the wanted name (see #499 (closed))
• configure DEBUSINE_DEFAULT_SCOPE to the wanted name (see !1141 (merged))
• start debusine

They need to be documented somewhere where people who manage debusine instances can find them.