Remove duplication of 3rd party doc

[mtrezza]

Issue

The docs for this package contain duplications of 3rd party docs, specifically APNS. See for example #174. It's unnecessary to copy/paste and maintaining that duplicated content.

Solution

Go through README.md and the files in /doc and replace duplications from the Apple docs with links to them. It can be various links per topic for easier navigation, or map node-apn options to APNS options , but no copy/pasted content from there.

[parse-github-assistant]

Thanks for opening this issue!