docs(gravity-forms-abilities): document productField auto-linking and add option field example

[faction23]

Description

Closes: N/A - documents plugin behavior from gravityforms#3716; no skills-repo issue exists.

• Document the new productField auto-link behavior. When an agent omits productField on a quantity or option field, current Gravity Forms links the field to the nearest preceding product field.
• Instruct agents to always set productField explicitly on forms with more than one product.
• Add the missing option field example row to the pricing table.
• Prepare the 1.1.1 release: add the changelog entry and bump the skill version to 1.0.4.

Testing instructions

1. Open skills/gravity-forms-abilities/references/field-config.md. Read the Pricing Fields section. Confirm the option example row and the productField note are present and correct.
2. On a site with the matching Gravity Forms branch, create a form through the MCP tools with one product and one quantity field, and omit productField. Confirm the quantity field links to the product, as the note describes.
3. Run ./scripts/validate-skills.sh. Confirm both skills validate.
4. Run ./scripts/release-notes.sh v1.1.1. Confirm the changelog entry prints.

Checklist:

• I've checked the error log to ensure my code doesn't throw any errors.
• My code follows the WordPress coding standards and inline documentation standards.
• My code follows the accessibility standards.
• This PR has a related issue.
• This PR has a related test.
• I have considered what will happen when an existing user updates to this branch.
• I have added a "Documentation Needed" label to this PR (if necessary).