Issue #3607685: Authored plain-text template bodies + README, requirements and docs

Frank Mablyrequested to merge
issue/orchestra-3607685:3607685-orchestraeasyemail-authored-plain-text into 1.x

The shipped Easy Email templates auto-derived their plain-text part from the HTML, which for the styled cards came out ugly (=== underlines, asterisks). This authors dedicated plain-text bodies (generateBodyPlain false) with the same tokens, adds a runtime requirements warning when no HTML mail backend is active, a README, and an Easy Email mention in docs/notifications.md. Follow-up to #3607508.

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