[docs] Notification batching documentation

[Dhanus3133]

Closes: #306

[nemesifier]

Good progress @Dhanus3133.

See my coments below.

I think we could move this information as a subsection of to https://github.com/openwisp/openwisp-notifications/blob/master/docs/user/web-email-notifications.rst, which seems to already touch the subject, @pandafy what do you think about this?

Please add a line mentioning this feature in https://github.com/openwisp/openwisp-notifications/blob/master/docs/user/intro.rst just under email notifications.

[pandafy]

I think we could move this information as a subsection of to https://github.com/openwisp/openwisp-notifications/blob/master/docs/user/web-email-notifications.rst, which seems to already touch the subject, @pandafy what do you think about this?

I agree here.

[pandafy]

I think we could move this information as a subsection of to https://github.com/openwisp/openwisp-notifications/blob/master/docs/user/web-email-notifications.rst, which seems to already touch the subject, @pandafy what do you think about this?

I agree here.

[nemesifier]

Good progress @Dhanus3133, please squash the commits so we have 1 commit for the CSS changes first, and the docs changes after, which I will rework.

See also my comment below.