docs: co-locate screenshots with markdown

[tito]

Summary

• Adds screenshots for the dashboard, LLM conversations, credential substitution, and header redaction docs under docs/img/.
• Updates the five referencing markdown files to use relative paths (./img/foo.png) instead of site-absolute paths.

Why

The previous sync (#main) brought in the markdown but referenced /img/greyproxy/foo.png, which only resolved on the docs site (where the PNGs were hand-placed under static/img/greyproxy/). Co-locating images with the markdown means:

• The docs-sync workflow (rsync -a --delete docs/ → docs/docs/greyproxy/) carries the screenshots automatically. No workflow change needed.
• GitHub renders the images correctly when browsing the markdown files in this repo.
• Docusaurus only routes .md / .mdx, so PNGs in the same folder do not appear as doc pages.

Test plan

• npx docusaurus build in the docs repo with the same layout (images at docs/docs/greyproxy/img/, relative refs) succeeds and pages render the screenshots.
• After merge, manually trigger the docs-sync workflow (or wait for the next release) and confirm the docs site renders the images.