fix(agentpool): [#169] add validation for name length when create_before_destroy is enabled

[matt-FFFFFF]

This PR replaces #170 to enable CI/CD checks with required secrets.

Original PR by @jcmendez-guerrero

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Description

This pull request improves the documentation and validation of the name input variable for agent pools, ensuring that naming conventions and constraints are clearly defined and enforced. Additionally, it introduces a configuration file for terraform-docs to standardize documentation generation.

Documentation improvements:

• Updated the description for the name input variable in README.md, modules/agentpool/README.md, and variables.agent_pool.tf to specify length and character requirements, and to clarify stricter rules when create_before_destroy is enabled. [1] [2] [3]

Validation enhancements:

• Added a new validation block in modules/agentpool/variables.tf to enforce a maximum length of 8 characters for the name variable when create_before_destroy is enabled.

Documentation tooling:

• Added .terraform-docs.yml to configure terraform-docs output, enabling consistent and automated documentation generation for the module.

Variable description update:

• Improved the inline description of the name variable in modules/agentpool/variables.tf to match the updated documentation.

Fixes #169

Type of Change

• Non-module change (e.g. CI/CD, documentation, etc.)
• Azure Verified Module updates:
  • Bugfix containing backwards compatible bug fixes
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates.
  • Breaking changes.
  • Update to documentation

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• I did run all pre-commit checks

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