CPLAT-9778: Update builder readme to reflect new codegen

[greglittlefield-wf]

Motivation

The builder/README.md file wasn't updated when we originally implemented the new parsing/codegen to support the new mixin-based boilerplate.

Goals:

• Update the document to reflect the new builder logic

Non-Goals

• Revamp that document entirely
• Go through parsing/codegen for every kind of declaration in detail

Changes

• Update the document to reflect the new builder logic
• Update generated code snippets
• Update outdated boilerplate in snippets

Release Notes

Review

See CONTRIBUTING.md for more details on review types (+1 / QA +1 / +10) and code review process.

Please review:

QA Checklist

• Markdown renders well
• Links aren't broken

Merge Checklist

While we perform many automated checks before auto-merging, some manual checks are needed:

• A Client Platform member has reviewed these changes
• There are no unaddressed comments - this check can be automated if reviewers use the "Request Changes" feature
• For release PRs - Version metadata in Rosie comment is correct

[aviary3-wk]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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Questions or Comments? Reach out on Slack: #support-infosec.

[corwinsheahan-wf]

Just a couple small things

[corwinsheahan-wf]

QA +1

• CI passes (docs only change)
• All links in changed files work and point to the correct locations.

Merging into master.
@Workiva/release-management-p

[rmconsole-wf]

+1 from RM