Consider adding a layout grid or guides to the site editor

[beafialho]

I decided to open this issue but let me know if this has been raised elsewhere.

Landing in a blank canvas is great for writing, but when you're designing a site or theme, visual guidance is desirable. Hence, a grid you could toggle on/off would be neat to have, given the responsive nature of the site editor.

A quick example of what customisation features it could contain:

• Number of columns
• Gutter size
• Outer margin size

I also made a quick visual example:

cc: @henriqueiamarino

[mtias]

See also #16271.

[henriqueiamarino]

The addition of grids on the Editor will require different views and grid variation.
Today we have those views under Preview as Desktop, Tablet and Mobile.

I think Webflow does a good work gathering both topics with simplicity — although they allow editing on a wide range of views, which can be confusing to non-designers. This might be a good initial inspiration here.