docs(storybook): fixes a number of accessibility issues in the documentation

[damienrobson-sage]

Proposed behaviour

The documentation offered by the storybook should be as accessible as possible. This PR aims to implement that by addressing a handful of issues:

• Changes the default secondary colour of the manager to the DS' Sage Green
• Fix the role assigned to the welcome page's headings
• Remove the duplicated header elements
• Add ARIA labels
• Add image alt text
• Replace blank table headings

Current behaviour

Checklist

• Commits follow our style guide
• Related issues linked in commit messages if required
• Screenshots are included in the PR if useful
• All themes are supported if required
• Unit tests added or updated if required
• Playwright automation tests added or updated if required
• 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
• 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

The following issues are embedded within Storybook and can't be directly addressed:

• The Create a new story button throws a "Buttons must have discernible text" warning,
• The "Zooming and scaling" failure is due to Storybook's meta tag configuration which we can't override (so I've raised an issue with them to fix it)
• The accessibility control panel causes 4 landmark violations

Testing instructions

Install Axe DevTools if not already installed. Open the documentation and then open the browser's inspector and switch to the DevTools tab. Run a full-page scan.