Analytics endpoints are undocumented

[PhilippTheServer]

The API gained four admin analytics endpoints (OpenTaberna/fastapi#44) and the wiki does not
mention them. Refreshing openapi.snapshot.json makes the drift check fail, which is the
check working as intended:

$ python3 tools/check_wiki.py
Checked 8 pages against 30 API paths.

4 problem(s):

  FAIL  API serves `/v1/admin/analytics/funnel`, but no wiki page mentions it
  FAIL  API serves `/v1/admin/analytics/products`, but no wiki page mentions it
  FAIL  API serves `/v1/admin/analytics/summary`, but no wiki page mentions it
  FAIL  API serves `/v1/admin/analytics/timeseries`, but no wiki page mentions it

Expected once solved

1. All four endpoints documented in the endpoint reference, with the metric definitions —
   what counts as revenue, and what does not.
2. The stated limits carried across, since a reader will otherwise assume something
   stronger: the funnel is an order funnel and not a visitor funnel, partial refunds are
   not modelled, and per-SKU return rate is an upper bound.
3. The new SHOP_TIMEZONE setting in the configuration reference.
4. openapi.snapshot.json refreshed and tools/check_wiki.py passing.