docs: fix broken links and update documentation

[AaronFeledy]

Changes

• Fix Netlify deploy preview failures: Added docs.backdropcms.org to the checklinks skipPatterns in netlify.toml. The Backdrop CMS docs site returns HTTP 406 to bot user-agents, causing the Netlify checklinks plugin to fail on every deploy preview.

• Update outdated PHP example: Changed the PHP version example in docs/config.md from 5.5 to 8.3 to reflect current recommended versions.

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Note

Low Risk
Docs-only changes plus a Netlify deploy-preview configuration tweak; no runtime or production behavior changes.

Overview
Updates docs/config.md to modernize the PHP configuration example from 5.5 to 8.3.

Adjusts netlify.toml deploy-preview link checking to skip docs.backdropcms.org, avoiding false failures when the external Backdrop docs site blocks automated requests.

^{Written by Cursor Bugbot for commit eca8fb7. This will update automatically on new commits. Configure here.}

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