Issue #3593605: document planned enhancements in the roadmap

Frank Mablyrequested to merge
issue/pdv-3593605:3593605-meta-pdv-roadmap into 1.x

List the planned pdv enhancements in docs/roadmap.md, grouped by theme and linked to their tracking issues, and point the roadmap at the meta. Add 'eca' to the project cspell dictionary for the new notification-convention entry.

Closes #3593605

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