refactor mailer module, add module docs

[akbarkz]

Related issues

#1812

Short description and why it's useful

The Mailer module has been refactored to comply with the latest module requirements. Also sendEmail method has been generalized to send any kind of emails. Docs for the module are also provided.

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

Contribution and curently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and I am aware that every new feature should be a module

[filrak]

Perfect example of module! Thanks @akbarik

[filrak]

Do you want to do some additional work? Otherwise let's merge it to develop

[akbarkz]

Do you want to do some additional work? Otherwise let's merge it to develop

No, I didn't plan to do something else with that :)

[filrak]

I'll merge then. Thank you @akbarik !. Maybe now you can try to make Notification and user module (for auth and user data)? We want to stay abckward compatible even if we change APIs with strangler pattern. You can see examples of porting new API to old components in core/components -> Microcart component