docs: add CLI docs style guide references and contributor guidance

Description

This MR makes the gitlab-org/cli project aware of the new CLI documentation style guide, recently published in gitlab-org/gitlab (gitlab!230899 (merged)).

It updates four files to surface the style guide's conventions:

• At the point of authoring (CONTRIBUTING.md).
• At review time (mr-review-instructions.yaml and Documentation.md).
• When agents generate or review CLI docs (AGENTS.md).

Changes and commits

• docs: add cli documentation style guide reference to CONTRIBUTING.md:
  • Clarifies the existing guidance on documentation MRs and adds a new "Documenting commands" section pointing contributors to the style guide.
• docs: add reviewer checklist to Documentation MR template:
  • Adds a pre-merge checklist to .gitlab/merge_request_templates/Documentation.md covering common docs issues:
    • short description conventions
    • synopsis requirements
    • experimental/beta feature handling
    • flag descriptions
    • examples
• docs: add documentation conventions to AGENTS.md:
  • Adds a "Documentation conventions" section so agents are aware of the rules when generating or reviewing CLI documentation, with a link to the full style guide.
• docs: add documentation conventions to mr-review-instructions.yaml
  • Adds a new Command documentation block scoped to internal/commands/**/*.go so that Duo code review surfaces documentation issues at the source, where CLI docs content is authored.

Related issues

Closes #8254 (closed)