Provide mockups for Service accounts

[vviers]

Following the plan outlined in https://github.com/gristlabs/grist-core/issues/1217, we agreed that designs would help us think the feature through.

There's a "UI/UX considerations" paragraph in the document

[lusebille]

Link to he mocks https://www.figma.com/design/wcpetFt6aOKzTszcvPPWLQ/%5B05%2F24%5D-Grist-Design?node-id=1711-67466&t=FBy2lCrLJIO56R0K-1