GTK+: Update the grid layout section

Rename the section Adaptive Grid Layout as there is no point to explain
how to set up a regular grid layout, and actually explain how to build
such a layout.
1 job for adaptive-grid-layout in 43 seconds (queued for 13 minutes and 23 seconds)
Status Job ID Name Coverage
  Test
passed #2442
librem5
job

00:00:43