Relative Links from content/index.md 404 when using basePath

[pcorliss]

Describe the bug

When the content/index.md file contains a relative link (./foo/bar.md) and basePath is set the link targets /foo/bar instead of /base-path/foo/bar. When clicking on the link the user receives a 404.

To Reproduce Steps to reproduce the behavior:

Repro repo: https://github.com/pcorliss/nextra-sidebar-repro

1. git clone https://github.com/pcorliss/nextra-sidebar-repro.git
2. cd nextra-sidebar-repro
3. npm ci
4. npm run dev
5. Navigate to http://localhost:3000/sidebar-repro
6. Clicking [relative link](./generated/aa/test-file-081.md) results in a 404
7. Clicking [relative with basePath](./sidebar-repro/generated/aa/test-file-081.md) renders
8. Clicking [absolute link](/generated/aa/test-file-081.md) renders

Expected behavior

Relative links from the content/index.md page should not 404.

Desktop (please complete the following information):

• OS: MacOs & Windows
• Browser: Chrome & Firefox
• Version Chrome 137.0.7151.104, Firefox 139.0.4 (aarch64)