This introduces a guide for extension authors on how to create columns

for the Dashboard and other datagrids. This walks through creating a

simple column and layering on custom rendering, linking, sorting, and

queryset augmentation, along with registration.
The DashboardColumnsHook and DataGridColumnsHook pages have been

modernized to reflect how columns are created now, and to refer to the

new guide for details on column creation.

Built the docs. Checked for any obvious build or link errors.