[Bug] Image directive doesn't support absolute paths

[theletterf]

Describe the bug

It looks like the image directive doesn't support absolute paths, only relative.

Expected behavior

I would expect the directive to be able to use absolute paths.

Steps to reproduce

Create an image directive and set the path to absolute.

Tooling

• docs-builder
• migration tooling
• I'm not sure

[theletterf]

+CC @colleenmcginnis and @bmorelli25 who had to fix a bunch of these here: elastic/opentelemetry#343

I wonder if the same happens with the snippets directive.