[Docs] Document dashboard and dashboard widgets

Add the following to the documentation:

• User docs
  • What is the dashboard
  • What are dashboard widgets
  • Customising the personal dashboard
• Admin docs
  • How are dashboard widget types provided
  • Creating dashboard widgets
  • Preferences concerning the dashboard
• Dev docs
  • How to create new dashboard widget types

added part::docs label

changed the description

assigned to @hansegucker

created merge request !918 (merged) to address this issue

mentioned in merge request !918 (merged)

closed with merge request !918 (merged)

mentioned in commit eaa37382

marked the checklist item User docs as completed

marked the checklist item What is the dashboard as completed

marked the checklist item What are dashboard widgets as completed

marked the checklist item Customising the personal dashboard as completed

marked the checklist item Admin docs as completed

marked the checklist item How are dashboard widget types provided as completed

marked the checklist item Creating dashboard widgets as completed

marked the checklist item Preferences concerning the dashboard as completed

marked the checklist item Dev docs as completed

marked the checklist item How to create new dashboard widget types as completed