v14- restructuring notification articles

[alina-tincas]

Description

What did you add/update/change?
Restructuring notification articles:

• most articles had information already about them on API docs, so I removed those articles and added a link to API docs on the readme file. Made a CMS PR to add extra information that was on docs but not on API docs: Update v14 api docs with info from umbraco docs Umbraco-CMS#15977
• added descriptions to the articles that were missing them
• added articles that existed under using notifications but not mentioned in the readme file
• moved some content from readme file to a new articles so that the readme file is showcasing an overview of all articles that "live" under it.

Type of suggestion

• Typo/grammar fix
• Updated outdated content
• New content
• Updates related to a new version
• Other

Product & version (if relevant)

cms v14, let me know if it makes sense to make these changes for the other versions as well?

Deadline (if relevant)

When should the content be published?
anytime

[jonat123]

@alina-tincas LGTM 💪