docs: document cache-relevant config fields

[Jonas Elfering (keulinho)]

Summary

• Documents the cache-relevant="true" attribute for plugin config.xml fields.
• Shows that the attribute can be used on <input-field> and <component> elements.
• Adds an app configuration cross-reference because apps use the same config.xml schema.

Related links

• Core PR: fix: respect cache-relevant system config fields shopware#17413
• Issue: Silent flag support for plugin configuration shopware#17367

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

Validation run locally:

• git diff --check
• make spellcheck
• targeted markdown-lint for the two changed files

No redirects were needed because no pages were moved, renamed, or deleted. .wordlist.txt was not changed because spellcheck passed without new terms.