docs: move notifications docs here

[saraburns1]

Moving documentation from https://github.com/openedx/openedx-platform/tree/master/openedx/core/djangoapps/notifications/docs to docs site for easier access/searching

[sarina]

Before merging can you do the following?

• Update tags in the doc (https://docs.openedx.org/en/latest/documentors/references/doc_style_guide.html#tags)
• Update the maintenance chart (https://docs.openedx.org/en/latest/documentors/references/doc_maintenance.html#maintenance-chart)

And then after merging:

• Update the docs in the repos being moved to point to the moved docs

[saraburns1]

tags are already set to .. tags:: site operator, how-to -- should it be something else?

[sarina]

@saraburns1 nah sorry I missed that I wasn't looking at the top of the doc. Tags look fine.

#1441