Docs: DO-3248 docs fixes readme and components docstrings

[patricia-causalens]

Motivation and Context

Fixing a number of docs issues found. Examples that didn't work, packages description being out of date after dara-ui repo was merged, images that were missing from the component docs.

Implementation Description

While looking into it found that we couldn't import ComponentItem from dara.components, also found an issue where CheckboxGroup could not have value as None which goes against the pattern for all other components. Both of these were also fixed.

Any new dependencies Introduced

How Has This Been Tested?

Locally on a test app to check if examples were working

PR Checklist:

• I have implemented all requirements? (see JIRA, project documentation).
• I am not affecting someone else's work, If I am, they are included as a reviewer.
• I have added relevant tests (unit, integration or regression).
• I have added comments to all the bits that are hard to follow.
• I have added/updated Documentation.
• I have updated the appropriate changelog with a line for my changes.

Screenshots (if appropriate):