update documentation regarding "integrations button"

[HarHarLinks]

https://github.com/turt2live/matrix-dimension/blob/08bd1319c5bf11fbba44ae5552125756f24e540f/docs/installing.md#L147

4 squares are only shown when there already are widgets.

https://github.com/turt2live/matrix-dimension/blob/7be069b3190c937d2f5b988c390e564bfcba6285/web/app/home/home.component.html#L241

3x3 I can't find at all.

[TheTimeWalker]

Duplicate of https://github.com/turt2live/matrix-dimension/issues/436

[HarHarLinks]

yes, but note there's a place this is referenced that isn't in 436.