docs(fulfillment): align rendering contract with totals schema

[juanferrub]

Summary

This updates the fulfillment rendering prose to match the existing schema and examples.

Specifically, it:

• changes the documented fulfillment option field from total to totals
• removes the unsupported statement that the price may be null
• updates the nearby rendering guidance so it no longer assumes a scalar total

Please let me know if I'm missing something, but it looks like the schema and examples consistently use totals, so this PR treats that as the source of truth and aligns the prose with it.

Why

Issue #559 points out that docs/specification/fulfillment.md documents groups[].options[].total as a required nullable scalar, while the actual schema requires groups[].options[].totals as an array of non-null amount entries.

The schema and examples already agree with each other, so this PR keeps the change intentionally narrow and only fixes the prose.

Validation

I triple-checked this in the local repo with the project tooling:

• scripts/test_validate_examples.py -> 43 passed, 0 failed
• scripts/validate_examples.py --schema-base source/schemas/ -> 270 passed, 0 failed, 0 errors, 45 skipped
• scripts/check_links.py site -> passed, all internal links valid
• git diff --check -> passed

Build note

I also checked the stricter docs build path locally. The full strict MkDocs build still aborts on pre-existing local-environment warnings, especially repeated cairosvg / missing libcairo warnings on this Mac. I verified those warnings reproduce on clean main as well, so they do not appear to be introduced by this change.