Restructure the documentation to make it easier to add new pages

[claremacrae]

Description

NB with all moves, remembered to add the alias to redirect old page to new - see Contributing guide

• [ ] Experiment with moving these pages to new Concepts section - or maybe Tasks Data??

• [x] Move these plugin pages to 'Other Plugins' section - maybe include Plugin/plugin in the file name???

• [ ] Divide up Daily Agenda page in to pages for separate plugins
• [ ] Maybe move Daily Agenda page to concepts???
• [ ] Move Notifications page to Other Plugins/Reminders plugin

Required Steps

No response

How to Test

No response