[v2] 404.html routePath does not include siteConfig.baseUrl

[cmfcmf]

🐛 Bug Report

The routesPaths parameter passed to the postBuild lifecycle event contains the wrong path to the 404.html file if you have a baseUrl configured in docusaurus.config.js.

This results in wrong data generated by plugins that access the routesPaths parameter. An example is the sitemap plugin, which generates an invalid sitemap entry.

Have you read the Contributing Guidelines on issues?

Yes :)

To Reproduce

1. Set the baseUrl to something different than just / in docusuaurus.config.json.
2. Build the documentation and log the routesPaths parameter in a plugin's postBuild lifecycle callback. You'll see the wrong path to the 404 file.
3. Open the generates sitemap. Again, the path to the 404 file is wrong.

Expected behavior

I would expect the routesPaths to look like this:

[ 
'/base-url/404.html',
'/base-url/',
'/base-url/docs/getting-started',
]

and the sitemap to contain:

<!-- ... -->
<url>
<loc>https://example.com/base-url/404.html</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<!-- ... -->

Actual Behavior

Currently, routesPaths look like this:

[ 
'404.html',
'/base-url/',
'/base-url/docs/getting-started',
]

The sitemap.xml file looks like this:

<!-- ... -->
<url>
<loc>https://example.com/404.html</loc>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<!-- ... -->

Reproducible Demo

To reproduce, build the documentation inside the docs folder of https://github.com/cmfcmf/OpenWeatherMap-PHP-API.