♻ [Placeholder] Add back subdomain support

Datahub supports arbitrarily nested domains.

In earlier versions of Find MoJ data we showed 2 levels: domain and subdomain. This allowed us to organise metadata first by organisational unit (e.g. HMCTS) and then have finer grained subject areas within each one (e.g. different kinds of courts)

Later, we swapped this out for a single-level domain model. However, since this is not testing well, and Scottish Gov have also mentioned using subdomains, I'm proposing we add subdomains back again in some form.

This is a placeholder ticket to record what needs doing if/when we revisit this.

Value of a hierarchical filter

we can restrict the top level subject area to a small number to avoid overwhelming users with choice
it allows the user to be either vague or specific when filtering by subject area

Current design: filter by top level domain

Behaviour

there is a single list of options
domains only get displayed as an options if they are top-level
if subdomains are used, certain entities will be inaccessible by browse / filtered search
- this behaviour was introduced when we switched to the listDomains query, which excludes subdomains (https://github.com/ministryofjustice/find-moj-data/pull/597)
- version b513e90 includes subdomains but doesn't display counts on the homepage

Previous design: Domain/Subdomain

Behaviour

if the user selects only the top level domain, the results include entities assigned to that domain or any of its subdomains
- this relies on code that has since been removed: https://github.com/ministryofjustice/find-moj-data/blob/bc02e954b3e98a9de70cca606d22a000b87cafc8/home/service/search.py#L22
users can refine the search further by choosing a subdomain
if datahub contains sub-sub-domains, these get ignored in the frontend
- but entities are still accessible when filtering on the domain/subdomain
javascript for this component is here: https://github.com/ministryofjustice/find-moj-data/blob/main/static/assets/js/enhanced-search.js
- if javascript is not available, then only the top level filter is shown

Definition of Done

[ ] Decided what to call the subdomain field
[ ] Decided whether to change the home page list
[ ] Added a feature toggle and documented in README
[ ] Updated the user guide
[ ] Ensured the tests cover with and without the feature flag

ministryofjustice / find-moj-data