docs: audit and fix missing language tags in markdown code blocks

[vanshika2720]

What this PR does:
Audits all active (non-versioned) documentation markdown files under docs/content/en/ and adds the appropriate language syntax highlighting tags (bash, yaml, hcl, terraform, console, text) to all code blocks that previously used plain untagged backticks (```).

Specifically, this PR fixes 28 code blocks across 9 active documentation files:

• control-plane-on-ecs.md (HCL/Terraform, YAML configs, bash commands)
• pipecd-at-kubecon-eu-2024.md / pipecd-at-kubecon-hongkong-2024.md (Plain text schedule metadata)
• metrics.md (Port forwarding and Helm CLI commands)
• command-line-tool.md (pipectl CLI examples)
• script-run.md (YAML snippets and folder structure text)
• auth.md (Access policies text)
• runtime-options.md (CLI help console output)
• contributing.md (Commit message examples)

Why we need it:
Without specifying language tags, code blocks render as unstyled plain text on the generated documentation site. Adding appropriate syntax tags enhances readability, visually structures the commands, and provides a polished developer experience for users reading the documentation.

Does this PR introduce a user-facing change?:

• How are users affected by this change: The code blocks in the documentation on the PipeCD website will now render with proper syntax highlighting.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[vanshika2720]

@rahulshendre @khanhtc1202 @eeshaanSA