Issue #3593894: document the pdv_eca submodule

Follow-up to the merged pdv_eca work: documentation was missing.

Adds a PDV ECA section to docs/submodules.md (the PDV: vault event, its channel/action filters, the full token table incl. [event:owner_langcode], and a dormancy-warning example), cross-links it from the PDV Mail section, and documents that the dormancy mail body is plain text an HTML-first mailer renders into paragraphs and links.