fix(v2): linking to asset or external html page -> don't use history.push()

[slorber]

Motivation

We should be able to use the existing webpack file-loaders when linking to assets in the static folder, and we should be able to link using the @site webpack alias.

We should only navigate to a markdown link using history.push() if that link is a known SPA route. We should not "navigate" to a pdf file (#3337), or an external HTML file that is not part of the SPA (#3309)

In the future, maybe the Link component should be able to know if a path like /javadoc is internal or external by matching it to known routes, but I don't think it's ideal either because it has a runtime cost, and it would also opt-out of broken link detection, as unknown routes would be considered as external and bypass this check.

The pathname:// protocol is an escape hatch that permits to tell Link that a link is expected to be considered as external. It is more explicit.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Unit tests

Dogfooding on a dedicated page:

• https://deploy-preview-3347--docusaurus-2.netlify.app/classic/markdown-tests/
• http://v2.docusaurus.io/markdown-tests

[docusaurus-bot]

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