[9.1](backport #47614) dev-tools: Refactor DocsDir to use git root directory

[mergify]

Proposed commit message

This fixes packaging errors in cloudbeat: elastic/cloudbeat#3567 by removing the assumption that we're following elastic/beats' structure.

Changes DocsDir() signature to return (string, error).

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Related issues

• Relates Bump beats version cloudbeat#3567
• Relates [documentation] update autogeneration scripts to generate markdown #44638

  ---

  This is an automatic backport of pull request dev-tools: Refactor DocsDir to use git root directory #47614 done by Mergify.

[github-actions]

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• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)