Init: repository governance, contribution templates, docs scaffold, and markdown CI

[csecrestjr]

Motivation

• Establish a baseline of repository governance, contribution guidance, and community standards for the NTARI Developer Portal.
• Provide issue and PR templates to improve contributor signals and triage.
• Add a Markdown-first documentation scaffold for the Engineering Handbook to reserve structure for future content.
• Improve authoring quality by adding editor and linting rules and repository ignore entries.

Description

• Added contributor and governance files including CONTRIBUTING.md, CODE_OF_CONDUCT.md, SECURITY.md, and updated README.md to reflect repository scope.
• Added repository metadata files CODEOWNERS, .editorconfig, and .gitignore to standardize formatting, ownership, and ignored files.
• Added issue templates under .github/ISSUE_TEMPLATE/ (bug_report.yml, feature_request.yml, documentation_improvement.yml, engineering_rfc.yml) and a PULL_REQUEST_TEMPLATE.md.
• Added CI workflows to lint and validate markdown with markdown-lint.yml, broken-link-check.yml, and spellcheck.yml, and scaffolded a suite of placeholder handbook pages under docs/engineering-handbook/.

Testing

• No automated tests were executed as part of this change.
• CI jobs were added for future runs: markdown-lint.yml, broken-link-check.yml, and spellcheck.yml, which will run on future push/pull_request events and scheduled checks.
• Local verification consisted of a quick review of added files for formatting and path correctness and no CI failures were observed because workflows have not yet run.

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Codex Task