orchestra_easy_email: authored plain-text template bodies + README, requirements and docs

Follow-up to #3607508 (the Easy Email channel).

The shipped Easy Email templates set generateBodyPlain: true with an empty plain body, so the plain-text alternative is auto-derived from the HTML. For the styled table-based cards that comes out ugly (headings become === underlines, strong becomes *asterisks*, etc.). Author dedicated plain-text bodies instead.

1. Custom plain-text bodies for the six shipped templates (set generateBodyPlain: false and author bodyPlain with the same tokens):

• orchestra_easy_email: orchestra_generic, orchestra_task_assigned, orchestra_task_reminder, orchestra_interaction_link
• orchestra_webform_example: orchestra_request_processed, orchestra_request_rejected

The subscriber already renders the plain part with sanitize off, so tokens ([orchestra-notification:link], [orchestra:var:validation.comment], etc.) come through unescaped.

2. Documentation (orchestra_easy_email currently ships no README):

• A hook_requirements (runtime) warning when no HTML-capable mail backend is active, so a site builder is told that Easy Email is delivered as plain text unless an HTML mailer is configured, rather than discovering it the hard way.
• A README.md describing the module, the orchestra_{type} template convention with the orchestra_generic fallback, the [orchestra-notification:*] tokens, and the HTML-mail-backend requirement.
• A short mention in docs/notifications.md.