Fix root-level 404 handling in multiversion Sphinx docs deployment

[AlgoFoe]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

• GitHub Pages only serves 404.html from the root of the gh-pages branch. While each documentation version correctly generates its own 404.html, the root-level file was not being updated during deployment.

What does this PR do?

• It updates the deploy_sphinx_docs_multiversion action to copy the built version’s 404.html file to the root of the gh-pages branch during deployment.

References

• Fixes 404 page at the root level is not updated #144.

How has this PR been tested?

• Verified that the copy operation runs only if the versioned 404.html exists.

Is this a breaking change?

• No.

Does this PR require an update to the documentation?

• No.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit