fix(plugins): trigger plugin hooks when saving plugin settings

The introduction of setPrivateSetting led to failing to trigger the
plugin hooks.

Fixes #6820, #7502
docs(features): added a link to the Elgg showcas page

ref: #6812
docs(plugins): moved the plugin skeleton documentation

ref: #6812
docs(views): moved the page structure best practices page from docs

ref: #6812
Merge pull request #7505 from jeabakker/plugin-settings-save

fix(plugins): trigger plugin hooks when saving plugin settings
docs(plugins): moved the dashboard plugin description from docs

This describes the functionality of the dashboard plugin

ref: #6812
docs(plugins): moved the file plugin description from docs

This describes the functionality of the file plugin

ref: #6812
docs(plugins): moved the groups plugin description from docs

This describes the functionality of the groups plugin

ref: #6812
docs(plugins): moved the profile plugin description from docs

This describes the functionality of the profile plugin

ref: #6812
docs(plugins): moved the pages plugin description from docs

This describes the functionality of the pages plugin

ref: #6812
docs(plugins): moved the messageboard plugin description from docs

This describes the functionality of the messageboard plugin

ref: #6812
docs(plugins): moved the messages plugin description from docs

This describes the functionality of the messages plugin

ref: #6812
docs(plugins): moved the blog plugin description from docs

This describes the functionality of the blog plugin

ref: #6812
docs(plugins): moved the thewire plugin description from docs

This describes the functionality of the thewire plugin

ref: #6812
docs(plugins): completed the list of bundled plugins

This adds a nice todo on which plugins are missing documentation

ref: #6812
docs(guides): moved the page handler documentation from docs

ref: #6812
docs(guides): moved the helper functions page from docs

ref: #6812
docs(guides): moved the context documentation from docs

ref: #6812
docs(guides): moved the plugin (user)settings documentation from docs

ref: #6812
docs(guides): moved the permissions check documentation from docs

ref: #6812