chore: Improve headings and guidelines throughout documentation

[VincentSmedinga]

Describe the pull request

What

• Uses sentence case for headings, instead of title case and documents this in Storybook contribution guidelines
• Changes ‘How to use’ and ‘Usage’ to ‘Guidelines’ for consistency, where appropriate
• Removes ‘When to use’ and ‘What to avoid’ for now – this distinction will return but more clearly and consistently
• Marks up guidelines as a list everywhere
• Move some instances of text from the ‘Examples’ section to ‘Guidelines’
• Fixes heading levels where necessary
• Increases consistency in Story names and headings
• Improves consistency for examples of the as prop

Why

The main goal was to stop using ‘Title Case’ in headings, even though it introduces a difference with the table of contents that Storybook generates, and to put the ‘Examples’ heading in the correct place for all components.

How

Manually :)

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• None

[dlnr]

Gotta love consistency.