Update migrated stories to conform to recommendations

[jamie-lynch]

What is the purpose of this change?

This PR updates the stories for components where documentation has already been migrated from zeroheight to conform to the recommendations agreed in #903

What does this change?

• Update the stories file in Accordion to Accordion.stories.tsx
• Create a new TextArea.tsx file, exports only from index.tsx and update stories file to TextArea.stories.tsx
• Create a new TextInput.tsx file, exports only from index.tsx and update stories file to TextInput.stories.tsx

[jamie-lynch]

Based on a number of new things that we've learnt, the recommendations are likely to change significantly. I'm closing this PR now so that we can tackle updating everywhere in one go once the recommendations are updated.