Modernize and harden the cheatsheets repository

[antonflor]

What changed

• Replaced the generic README with a categorized repository index, safety guidance, maintenance policy, and accurate links.
• Added an MIT license, contribution guide, documentation style guide, and content-correction issue template.
• Added GitHub Actions checks for Markdown quality and documentation links on the maintained documentation surface.
• Rewrote the Cisco IOS/IOS XE and Arista EOS references to remove vendor-mixed commands and clearly separate read-only inspection from disruptive operations.
• Modernized Docker Compose v2, Terraform 1.x, Azure CLI, Google Cloud CLI, Kubernetes, iptables/nftables context, and REST API guidance.
• Replaced the misspelled kubernates.md with kubernetes.md.
• Replaced RESTful.md with the consistently named and substantially improved rest-api.md.
• Removed the employer-specific Datum interview dossier from the public general-purpose cheatsheet repository.

Why

The repository advertised regularly maintained, easy-to-navigate operational references, but it lacked an index, version markers, contribution standards, a license, and automated checks. Several high-use sheets contained stale syntax, unsafe defaults, obsolete terminology, or commands from the wrong vendor platform.

Impact

The repository is now easier to browse, safer to use during operations, more credible as a public portfolio project, and structured for incremental maintenance. Destructive commands are called out explicitly and maintained sheets include applicability and review metadata.

Validation

• Compared the branch against main: 19 files changed across 21 commits, with no branch divergence.
• Checked internal README paths against the resulting file set.
• Added Markdown and link-check workflows for the rewritten and newly maintained documents.
• Reviewed current terminology and command direction against official Docker, HashiCorp, Microsoft, Google Cloud, Kubernetes, Linux firewall, HTTP RFC, and OpenAPI documentation.

Follow-up boundary

This PR deliberately establishes a maintained core rather than pretending every legacy sheet has been freshly validated. Untouched legacy sheets remain indexed, but should be migrated to the same metadata and safety standard in future focused passes.