doc: fix three dead reference links

[blackorange-mega]

Three external references in the docs no longer resolve. All checked with a browser user-agent and following redirects.

File	Link	Status	Replacement
doc/en/how-to/output.rst	windyroad.com.au/dl/Open%20Source/JUnit.xsd	404	github.com/windyroad/JUnit-Schema/blob/master/JUnit.xsd
doc/en/explanation/flaky.rst	blogs.dropbox.com/tech/2016/03/open-sourcing-pytest-tools/	redirects to the dropbox.tech front page	Wayback snapshot
doc/en/explanation/flaky.rst	testandcode.com/50	404	Wayback snapshot

Notes:

• The JUnit XSD is still published in the windyroad/JUnit-Schema repository, so that citation keeps pointing at the actual schema rather than an archive.
• The Dropbox post is a soft failure — the URL still returns 200 but lands on the blog's front page, so the citation no longer reaches the article it names. Neither it nor the podcast episode has a live successor URL, so both use Wayback snapshots, matching what doc/en/history.rst already does for older dead references.
• Per CONTRIBUTING, no changelog entry since this is a documentation fix.

I also checked the other links flagged by a scan of the docs and left them alone: stackoverflow.com/search?q=pytest and bsky.app/profile/pytest.org in contact.rst both resolve fine (they only appear broken to non-browser clients), and the remaining dead ones are inside doc/en/announce/ release notes and history.rst, which are historical records.

[Pierre-Sassoulas]

LGTM, but let's remove the changelog

[blackorange-mega]

Done — changelog entry removed, the PR is now just the two documentation files. The Changelog entry check has gone from failing to neutral, so nothing is blocked on it.