test(blog-plugin): fix ability to generate proper blog website fixture build snapshot

[slorber]

As part of #9151, we found out that some blog test fixtures have been generated manually, and it's not clear how to regenerate them.

This PR adds a proper setup to generate the packages/docusaurus-plugin-content-blog/src/__tests__/__fixtures__/website/build-snap fixture folder, by building a real Docusaurus fixture site.

We remove the assets folder, we don't really need it.

Also, I replaced the docusaurus.png fixture image with a larger one to avoid large snapshots containing url-loader base64 inlined images. That makes snapshots harder to review for no added value.

No blog production implementation code was touched.

The build-snap folder can be regenerated with

yarn workspace @docusaurus/plugin-content-blog test:generate-build-snap

We will see later if it's worth enforcing build-snap does not become stale in CI (similarly to how we ensure yarn.lock is fresh with git diff --exit-code?)

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[github-actions]

Size Change: 0 B

Total Size: 1.1 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	117 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	829 kB
website/build/index.html	41.3 kB

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