#8509: docs: add docs CONTRIBUTING.md

This PR adds a CONTRIBUTING.md file to the docs/ directory with guidance on how to specifically contribute to the OpenClaw documentation. Goal is to help human and agent contributors successfully create high quality content with * How to build a local preview to check changes before creating a PR * Required frontmatter and file formats * Style and writing tips <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds `docs/CONTRIBUTING.md`, a documentation-specific contributing guide covering local preview workflow, required frontmatter, writing/style tips, MDX component usage, navigation updates, images, and a pre-PR checklist. This complements the repo-wide guidelines by giving doc contributors concrete Mintlify-focused instructions and conventions. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge; it adds documentation only with a few likely-to-confuse command/path details. - Change is a new docs-only file with no code execution impact. The main risks are correctness/usability of the contribution instructions (CLI invocation and config file path), which are easy to fix and won’t affect runtime behavior. - docs/CONTRIBUTING.md <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->