chore: update local docs build script + contributing instructions

[damaz91]

Description

• Updated the script to generate the docs locally in a single command (also fixing a broken link to "Specifications" in the left-side menu)
• Re-added repo-level CONTRIBUTING.md for better discoverability (will clean-up the org level CONTRIBUTING.md as a follow-up)
• Updated the CONTRIBUTING.md to include the build_local.sh instructions

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Checklist

• I have followed the Contributing Guide.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

[ptiper]

@damaz91 why are you adding this? The CONTRIBUTING.md is hosted in the .github repo, at the org level, shared across all repos: https://github.com/Universal-Commerce-Protocol/.github/blob/main/CONTRIBUTING.md

[damaz91]

This was intentional, IMO there are (at least) 2 good reasons to have a CONTRIBUTING.md at the repo level.

1. when using LLMs to code against the repo, having the CONTRIBUTING.md in the repo make it so much easier to discover it automatically

2. the CONTRIBUTING.md contains "ucp" main repo specific instructions (like the "Capability Maturity Level" or "Local development" section") which do not apply to any other repo