docs(dashboards): update to describe current behaviour of system

[mkschell]

Description

1. index.md (Dashboards overview)

• Removed the outdated "only available in MODX 2.2 or later" line (this is the 3.x branch)
• Added a new Customizable Dashboards section explaining the Customizable checkbox, per-user cloning, and what happens when it's unchecked
• Added a How Template Changes Propagate subsection explaining that widget additions/removals from the manager propagate to all users, while direct dashboard changes are per-user only

2. managing.md (Managing Your Dashboard)

• Added a Template vs Personal Changes section clarifying that edits on this page update the template, that additions/removals propagate to all users, and that direct dashboard page changes only affect the current user
• Included practical guidance on when to use each editing interface

3. usergroups.md (Assigning to a User Group)

• Fixed the intro line (it previously said "how to edit a Dashboard" which was copy-pasted from the managing page)
• Added a Customizable Dashboards and User Groups section explaining that personal copies are created on first login, and that subsequent template changes propagate

4. creating-a-widget.md (Creating a Dashboard Widget)

• Added a blockquote note after the "Assigning the Widget" instructions explaining the difference between adding a widget via the manager (template-level, affects all users) vs directly on the dashboard (per-user only)
• Included a performance note about placement records on large sites

Affected versions

3.x

Relevant issues

These are the issues that led me to creating this PR, but there are likely many more:

modxcms/revolution#14753
modxcms/revolution#15324