Document that workspace crates should use absolute image URLs in README

When a workspace crate references a root-level README via a relative path (e.g. readme = "../../README.md"), any relative image URLs in that README will not resolve correctly on crates.io. This is because crates.io resolves paths relative to the crate subdirectory, not the repository root.

The workaround is to use absolute URLs (e.g. https://raw.githubusercontent.com/{owner}/{repo}/HEAD/path/to/image.png), but this is not documented anywhere obvious. It's only discoverable by digging through closed issue comments:

• https://github.com/rust-lang/crates.io/issues/9939
• https://github.com/rust-lang/crates.io/issues/9886
• https://github.com/rust-lang/crates.io/issues/13376 (redirected here)

It would help to add a note to the package.readme section of the Cargo reference, e.g.:

Note: If your crate lives in a subdirectory and references a README outside the crate directory (e.g. readme = "../../README.md"), relative image URLs in that README will not resolve correctly on crates.io. Use absolute URLs for images instead.

Start at the Cargo reference's package.readme section, linked in the issue, and review how README paths are currently documented. Add a note explaining the workspace-crate case and recommending absolute image URLs, then verify that the reference clearly covers the example and crates.io behavior described in the issue.