doc/subscriber_guide: add Email notifications section

[JenySadadia]

Explain KCIDB email notifications.

[JenySadadia]

@spbnick Is developer guide the correct place to mention email notifications or should I add this section to the submitter guide?

[spbnick]

I think this kind of info needs a new file - the docs for the consumers of our information, that is kernel and test maintainers and developers, plus possibly CI developers and maintainers. The "Developer Guide" is for developers of KCIDB. Perhaps we should call it subscriber_guide.md.

[JenySadadia]

OK, I'll add a new page.

[spbnick]

Thank you, Jeny! This looks good. I would only like to see better formatting of the Markdown source, where possible, as its whole point (ok, maybe not the whole, but most) is to be readable as is, and long lines are not that readable.

And another nitpick: since the subscriber guide did not exist before your commit, the correct commit subject would look something like this: doc: Add a submitter guide.

[spbnick]

LGTM. Thank you, Jeny!