fix(v2): brokenLinks should not report links that belong to an existing folder if folder/index.html exists

[slorber]

Motivation

#3303

Let's consider the static folder:

We should be able to link to the /javadoc, and despite not being an SPA link, it should not be reported as a broken link.

Test Plan

Unit tests + test on Docusaurus website

[javadoc](/javadoc)

[javadoc](/javadoc/)

[javadoc](/javadoc/index)

[javadoc](/javadoc/index.html)

[javadoc](/javadoc/hey)

[javadoc](/javadoc/hey.html)

[javadoc](/javadoc/bad)

[javadoc](/bad)

[javadoc](/bad/)

Lead to the expected result:

And we don't report anymore the good javadoc paths

[docusaurus-bot]

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