yarn build error with "Docusaurus found broken links" after set baseUrl to "/docs/"

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Prerequisites

- [X] I'm using the latest version of Docusaurus.
- [X] I have tried the `npm run clear` or `yarn clear` command.
- [X] I have tried `rm -rf node_modules yarn.lock package-lock.json` and re-installing packages.
- [X] I have tried creating a repro with https://new.docusaurus.io.
- [X] I have read the console error message carefully (if applicable).

### Description

yarn build error with "Docusaurus found broken links" after set baseUrl to "/docs/"

### Steps to reproduce

1. init
```sh
npx create-docusaurus@latest docs classic
```

2. build
```sh
cd docs
yarn build
...
[SUCCESS] Generated static files in build.
[INFO] Use `npm run serve` command to test your build locally.
Done in 69.04s.
PS D:\Document\github\docs>
```

3. change baseUrl from '/' to '/docs/'
```diff
# docusaurus.config.js
- baseUrl: '/',
+ baseUrl: '/docs/',
```

4. build

```
yarn build
...
[ERROR] Unable to build website for locale en.
[ERROR] Error: Docusaurus found broken links!

Please check the pages of your site in the list below, and make sure you don't reference any path that does not exist.
Note: it's possible to ignore broken links with the 'onBrokenLinks' Docusaurus configuration, and let the build pass.

It looks like some of the broken links we found appear in many pages of your site.
Maybe those broken links appear on all pages through your site layout?
We recommend that you check your theme configuration for such links (particularly, theme navbar and footer).
Frequent broken links are linking to:
- /docs/intro

Exhaustive list of all broken links found:

- On source page path = /docs/404.html:
-> linking to /docs/intro
```

5. check the links in files generated by build
```html
Docusaurus Tutorial - 5min ⏱️
```

### Expected behavior

the links in files generated by build should be like this
```html
Docusaurus Tutorial - 5min ⏱️
```

### Actual behavior

link in document should be like '/docs/docs/intro', but actually is '/docs/intro'

### Your environment

- Public source code: https://codesandbox.io/s/zealous-meadow-i63wc
- Public site URL: https://codesandbox.io/s/zealous-meadow-i63wc
- Docusaurus version used: 2.0.0-beta.14
- Environment name and version (e.g. Chrome 89, Node.js 16.4): Chrome 94, Node.js v14.18.1
- Operating system and version (e.g. Ubuntu 20.04.2 LTS): win10x64

### Reproducible demo

_No response_

### Self-service

- [ ] I'd be willing to fix this bug myself.