Add "generic" fallback mentions to regular/digest template docstrings

Sumit_goyalrequested to merge
rjsumit71/postorius:add-generic-template-docs into master

This MR addresses the child issue of mailman#1243 regarding the documentation of template fallback behavior in Postorius.

Currently, list administrators are unaware that if they do not configure the regular or digest headers/footers, Mailman Core automatically falls back to a generic template.

This MR improves the visibility of this behavior within the Postorius UI by:

  1. Adding the missing list:member:generic:footer and list:member:generic:header templates to TEMPLATES_LIST.
  2. Appending an explicit warning to the descriptive docstrings of the regular and digest templates (e.g., "Falls back to list:member:generic:footer if not set.").
  3. Providing the generated Django migration (0020_alter_emailtemplate_name.py) to safely apply these docstring updates to the database schema.

Closes #639

Edited by Sumit_goyal

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