Docs: Add deprecation warning for src/config.js (Volto 18+)

[gummybearansh]

Description

Updates the configuration registry documentation to reflect that the project configuration approach using src/config.js is deprecated in Volto 18+ in favor of the config/ folder structure.

Related Issue

Closes #7601

Changes

• Added a {warning} block to docs/source/configuration/how-to.md regarding the deprecation.
• Added a link to the Volto Customization training for the new config/ approach.
• Clarified that src/config.js is still valid for older projects.

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📚 Documentation preview 📚: https://volto--7756.org.readthedocs.build/

[boring-cyborg]

Caution

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[mister-roboto]

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[stevepiercy]

Thanks for your contribution! I've made some suggestions.

[mister-roboto]

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[gummybearansh]

Thanks for your contribution! I've made some suggestions.

Thanks for the detailed review and the guidance on MyST directives!

I have updated the file to:

1. Revert the accidental header formatting.

2. Use the {versiondeprecated} and {file} directives as requested.

3. Remove the external link to the training materials to keep the docs as the source of truth.

I will open a separate PR shortly to add the .vscode/settings.json file as you suggested.

Ready for another look when you have a moment.

[stevepiercy]

LGTM, but I want a technical review from the Volto Team before it's merged.

Also before merging, we need your permission to publish your work. See https://6.docs.plone.org/contributing/#sign-and-return-the-plone-contributor-agreement.

[stevepiercy]

Also it needs a change log entry per https://6.docs.plone.org/contributing/#change-log-entry

[mister-roboto]

@gummybearansh you need to sign the Plone Contributor Agreement to merge this pull request.

Learn about the Plone Contributor Agreement: https://plone.org/foundation/contributors-agreement

If you have already signed the agreement, please allow a week for your agreement to be processed.
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[gummybearansh]

I have already signed and submitted the form. I am currently waiting for the acceptance confirmation

I have added the changelog entry.

Standing by for Volto team technical reivew.