chore: remove mail rendering

[BJTheCod3r]

This pull request removes the mail rendering abstraction (MailRenderer) from the notifications system. Now, mail messages are sent directly as provided, without a rendering step or renderer injection. The code and documentation are updated to reflect this simplification, and all related interfaces, constants, and tests for mail rendering are removed.

Removal of mail rendering abstraction:

• The MailRenderer interface and all related code, including the MAIL_RENDERER injection token and mail renderer provider option, are removed.
• The NodemailerMailAdapter no longer accepts or uses a renderer; it sends the mail message as received. (src/adapters/nodemailer-mail.adapter.ts
• All tests related to mail rendering are removed from the Nodemailer adapter test suite. (src/__tests__/nodemailer-mail-adapter.spec.ts

API and documentation updates:

• The toMail method in the notification interface and base class is now explicitly asynchronous, returning a Promise.
• Documentation is updated to remove references to mail renderers and clarify the simplified mail sending process.

[tobycodes]

LGTM