Tabularium 0.25.2 - Resolve relative README links

Fix relative links in plugin READMEs so they open files in the GitHub repository. Relative images now load from GitHub raw content, while internal anchors and absolute URLs remain unchanged.

Applies to both the main plugin README and release READMEs. Rendered README cache keys are refreshed to serve the fix immediately after deployment.

Validation: 28 focused tests passed, lint passed, and CI passed for the fix commit.