#20997: add cognee as community plugin

Add @cognee/cognee-openclaw as described in the community plugin listing Cognee memory plugin that auto-recalls relevant memories before agent runs and auto-indexes workspace memory files to Cognee <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds Cognee Memory as the first community plugin to the documentation. The entry follows the exact format specified in the template and includes all required fields (plugin name, npm package `@cognee/cognee-openclaw`, GitHub repository, description, and install command). <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is a simple documentation addition that follows the established template format exactly. It adds a community plugin entry with all required fields properly formatted. No code changes, no security concerns, and no breaking changes. - No files require special attention _{Last reviewed commit: 75562d0} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->