Add contributor documentation and clean up stale references

[frisodebacker]

Add CONTRIBUTING.md and DeveloperDocs/ with architecture, documentation system, peripheral guide, and testing reference. Update README.md to surface these resources and reflect the current peripheral set (SPI, TWI). Remove stale references to the deleted generate_avr_tools_device_file.swift from AGENTS.md and README.md. Rename splitTarget to splitTargetLSB in SUPPLEMENTAL_DOCUMENTATION.md to match the actual field name.

[Copilot]

Pull request overview

Adds contributor-facing documentation for HALGEN workflows and updates existing references to reflect current generator behavior and supplemental documentation fields.

Changes:

• Adds CONTRIBUTING.md and detailed guides under DeveloperDocs/.
• Updates README contributor links, generated peripheral list, and sample output tree.
• Renames supplemental documentation references from splitTarget to splitTargetLSB and removes stale script references.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Surfaces contributor docs and updates peripheral/output documentation.
docs/SUPPLEMENTAL_DOCUMENTATION.md	Updates split bitfield key references to splitTargetLSB.
DeveloperDocs/TESTING_AND_GENERATION.md	Adds build, generation, testing, and inspection command reference.
DeveloperDocs/DOCUMENTATION_SYSTEM.md	Adds supplemental documentation system guide.
DeveloperDocs/ARCHITECTURE.md	Adds architecture and generator lifecycle overview.
DeveloperDocs/ADDING_PERIPHERALS.md	Adds guide for adding or changing peripheral generators.
CONTRIBUTING.md	Adds onboarding workflow and review checklist.
AGENTS.md	Removes references to the ATDF model regeneration helper.

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