Using pathPrefix creates wrong hyperlinks in sidebar

### Steps to reproduce

1. Add `pathPrefix` config to `gatsby-config.js`
2. Build using `gatsby build --prefix-paths` ([Gatsby docs](https://www.gatsbyjs.org/docs/path-prefix/))

### Expected behavior

Paths should have the path prefix added once.
e.g. `/endpoints` would become `/my-path/endpoints` after build.

### Actual behavior

For assets and links inside `.mdx` pages inside `/docs` the path is added once, but for links created in `sidebar.mdx` the path is added twice.
Actual result: `/my-path/my-path/endpoints`

### Actual generated example

`src/gatsby-theme-documentation/sidebar.mdx`
```html

### `gatsby-config.js`

```js
module.exports = {
siteMetadata: {
title: 'API Documentation',
description: 'Docs for API',
},
plugins: [
'gatsby-theme-documentation',
],
pathPrefix: 'my-path',
}

```