prefix project-resolved logo paths with slash

[gordonwoodhull]

• fixes Brand: logo paths are resolved incorrectly for documents in sub directories #11982

This breaks a huge number of tests because any relative logo paths now get a leading slash.

Implements the messy but narrow change of prepending a slash to any logo paths only if the input document is not in the project directory. (If we don't know, we also prepend.)

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[gordonwoodhull]

It's messier code but safer to only prepend a slash when the document is not in the project directory, so I've implemented a shared helper function logoAddLeadingSlashes() and call it each place we call resolveLogo().

Where we know the input document directory we can compare it with the project directory, and only add the slash if they differ.

There are two places we don't know the input document directory:

• navbar/sidebar: these always prepended the slash, and still do
• the brand shortcode: that information is not available here, so it always prepends the slash

I changed the tests accordingly for the shortcode (for HTML it gets transformed to ./ in a post-processor).

Typst is the only remaining format suffering from

• Brand: logo paths are resolved incorrectly for documents in sub directories #11982

and this is because the typst brand logo bypasses Markdown and paths don't get transformed by project-paths.lua so I'm going to see if I can change that.

(The brand logo shortcode does work in Typst.)

[gordonwoodhull]

I'm going to go ahead and merge this, since the Typst problem is unrelated.