#2 #3 add membership_payment_date: membership start date -> payment date. Don't overlap membership periods on renewals

membership_payment_date/readme/USAGE.rst

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+#. Create a membership product (fixed or variable period).
+#. Sell the product to a member via a sales order or invoice.
+#. The invoice generates a membership line with provisional dates.
+#. When the invoice is paid (e.g. bank transfer registered manually),
+   the membership line dates are automatically updated:
+
+   * **date_from** is set to the payment date (or the day after the
+     current active membership expires, in case of renewal).
+   * **date_to** is recalculated based on the new date_from for
+     variable-period products.