feat: Add PROXY_URL environment variable description

[Max (aragon999)]

Summary

While fixing shopware/shopware#18687 I checked first the docs, what this variable is for, but did not find any documentation.

Related links

https://github.com/shopware/recipes/blob/f282a2222e0a1f1cc460152f6b287f2eca44cfe3/shopware/storefront/6.7/manifest.json#L11

Checklist

• I reviewed affected links, code samples, and cross-references, including PageRef references where relevant.
• I added or updated redirects in .gitbook.yaml if pages were moved, renamed, or deleted.
• I updated .wordlist.txt (and sorted it) if spellcheck flags new legitimate terms.
• Any required dependent changes in downstream modules have already been merged and published.
• This pull request is ready for review.

Notes

[shopware-dev-docs-connector]

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Repository: aragon999/docs
Commit: 06c1b1e
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Last approval checked: Su (@sushmangupta)

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[Su (sushmangupta)]

Thanks for the PR Max (@aragon999) :)