feat(preview): add `shape` and `disableAnimation` props

[nuria1110]

Proposed behaviour

• Adds shape prop - "text", "rectangle", "rectangle-round" and "circle".
• Adds the disableAnimation prop to disable the loading animation, this can be set manually or will automatically be true when prefer reduce-motion is enabled.
• Updates background colour to a gradient.

Current behaviour

• Preview does not have a shape prop.
• Preview does not handle prefer reduce-motion.

Checklist

• [x] Commits follow our style guide
• [ ] Related issues linked in commit messages if required
• [x] Screenshots are included in the PR if useful
• [ ] All themes are supported if required
• [x] Unit tests added or updated if required
• [x] Playwright automation tests added or updated if required
• [x] Storybook added or updated if required
• [ ] Translations added or updated (including creating or amending translation keys table in storybook) if required
• [ ] Typescript d.ts file added or updated if required
• [x] Related docs have been updated if required

QA

• [ ] Tested in provided StackBlitz sandbox/Storybook
• [ ] Add new Playwright test coverage if required
• [ ] Carbon implementation matches Design System/designs
• [ ] UI Tests GitHub check reviewed if required

Additional context

Testing instructions

• Shape variations should support changes to dimensions using width and height props.
• The "circle" shape uses height as its diameter, ignoring the width.
• The lines prop should render specified number of Preview shapes.
• When shape is "text", if more than one line, the last line should be 80% of the width.
• Prefer reduce-motion should remove animation.