docs: update line item ids to differentiate from item ids

[thompson-tomo]

Description

This goes through and updates the checkout page to increase consistency with the index page and reduces ambiguity as to what the line_items_id refer to.

This is done by:

• line_items.[].id is always li_1 as per index page example and differentiates it from line_items.[].item.id which is item_123
• fulfillment.methods.[].line_items_id is always li_1 to reference line_items.[].id
• fulfillment.methods.[].groups.[].line_items_id is always li_1 to reference line_items.[].id

Type of change

• Documentation update

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

---

Pull Request Title

This repository enforces Conventional Commits. Your PR title must follow
this format: type: description or type!: description for breaking changes.

Types:

• feat: A new feature
• fix: A bug fix
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space,
  formatting, etc)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing tests or correcting existing tests
• chore: Changes to the build process or auxiliary tools and libraries

Breaking Changes:

If your change is a breaking change (e.g., removing a field or file), you
must add ! before the colon in your title:
type!: description

Examples:

• feat: add new payment gateway
• fix: resolve crash on checkout
• docs: update setup guide
• feat!: remove deprecated buyer field from checkout