Add Pulumi and spanning-tree guides and normalize filenames

[antonflor]

What changed

• Added a comprehensive pulumi.md covering projects, stacks, configuration, secrets, previews, deployments, imports, drift, state recovery, destruction, and CI/CD usage.
• Added spanning-tree.md covering STP, RSTP, MSTP, election logic, port roles and states, PortFast, BPDU Guard, Root Guard, Loop Guard, EtherChannel interaction, and troubleshooting.
• Merged gcpcloud.md and gcloud.md into google-cloud.md so Google Cloud concepts and CLI operations live in one maintained reference.
• Merged awscloud.md and awscli.md into aws.md for the same one-platform/one-reference structure.
• Renamed and substantially modernized:
  • github.md to git.md
  • leafspine.md to leaf-spine.md
  • linux_kernel_boot.md to linux-boot.md
• Updated the README, style guide, and documentation-quality workflow for the new files and naming convention.

Naming decision

The repository now prefers descriptive lowercase kebab-case filenames. Renames are intentionally limited to misleading, compressed, or underscore-based names; stable single-word filenames are not being churned for appearance alone.

Platform overview and primary CLI material are combined when they serve the same operational audience. Distinct infrastructure-as-code tools such as Terraform and Pulumi remain separate because their state, safety, and operational workflows differ.

Validation

• Branch is based directly on current main with no divergence.
• README and CI references were updated alongside every rename or merge.
• Maintained documentation structure and internal links will be validated by GitHub Actions.
• Pulumi, AWS CLI, Google Cloud CLI, and spanning-tree guidance was reviewed against official project and vendor documentation.