Fix assets not found in Express server

[endiliey]

Motivation

Fixes #600

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Create an image file in docs/assets/docusaurus.png

2. Paste this to docs/getting-started-installation.md content
   ![docusaurus](/docs/assets/docusaurus.png)

3. Run server on windows

cd website
npm start

http://localhost:3000/docs/en/next/installation.html

Before this PR

After this PR

Explanation:

According to https://expressjs.com/en/starter/static-files.html
The function signature is:

express.static(root, [options])

Previous code

  app.use(
    join(siteConfig.baseUrl, 'docs', 'assets'),
    express.static(join(CWD, '..', readMetadata.getDocsPath(), 'assets'))
  );
  app.use(
    join(siteConfig.baseUrl, 'blog', 'assets'),
    express.static(join(CWD, 'blog', 'assets'))
  );

We should not use path.join on Express root argument. Express.js expect something like /docs/assets/ as a root argument. path.join in Unix systems work differently when in Windows work.

Example:

console.log(path.join(siteConfig.baseUrl, 'docs', 'assets'));

In UNIX /docs/assets
In Windows \docs\assets

[docusaurus-bot]

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[docusaurus-bot]

Deploy preview for docusaurus-preview ready!

Built with commit 0c04019

https://deploy-preview-720--docusaurus-preview.netlify.com

[JoelMarcey]

❤️

Thank you @endiliey!