docs(progress-stepper): add horizontal width limitation example

[whitelisab]

Purpose of PR

This PR adds to the website documentation for the ProgressStepper component by adding an example explaining the extra width that appears on the component when rendered in the horizontal orientation. We added a new example with borders to visually demonstrate this, as well as an explanation.

Screenshot of changes to the website:

PR Checklist

• [x] I have read the relevant readme.md file(s)
• [x] All commits follow our Git commit message convention
• [x] Tests are added/updated/not required
• [x] Tests are passing
• [x] Storybook stories are added/updated/not required
• [x] Usage notes are added/updated/not required
• [x] Has been tested based on Contentful's browser support
• [x] Doesn't contain any sensitive information

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: a38356225cce8a80b3c8ca29035c38b6803844b7

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Name	Status	Preview	Updated (UTC)
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[github-actions[bot]]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
CommonJS	112.03 KB (0%)	2.3 s (0%)	1.5 s (+23.4% 🔺)	3.7 s
Module	110.84 KB (0%)	2.3 s (0%)	1.7 s (+3.66% 🔺)	3.9 s