Fix static HTML class list navigation path duplication (#1639)

[carsonclarkemagrab-toast]

Summary

Fixes #1639

This fixes a breaking bug in v0.9.38 where clicking nested class links in the class list generates incorrect URLs when serving static HTML documentation, resulting in 404 errors.

Root Cause

The fetch-based navigation system (commit e18b844) resolves relative URLs from the class list iframe against the main frame's current URL instead of the iframe's base URL. When viewing /Foo/Bar.html and clicking a link to Foo/Baz.html, the browser incorrectly resolves it as /Foo/Foo/Baz.html.

The Fix

• Resolve URLs against the iframe's contentWindow.location instead of the main frame location
• This ensures relative paths like Foo/Baz.html resolve correctly regardless of the current page

Testing

Reproduced the issue with nested class structure:

class Foo
    class Bar; end
    class Baz; end
end

• Before fix: Clicking Foo::Baz from /Foo/Bar.html → /Foo/Foo/Baz.html
• After fix: Clicking Foo::Baz from /Foo/Bar.html → /Foo/Baz.html

Related: block/elasticgraph#1048

[carsonclarkemagrab-toast]

Seems like I opened this right after a fix put up in another PR: #1646