Relative links in the Upcoming Releases page can break docs builds

[ptgott]

In the prepare-files.mts script that the docs engine runs before building the Docusaurus site, we copy the Upcoming Releases page from the default version of the docs to the non-default versions. One undesired effect of this logic is that, if an author adds a relative link to a docs page to the Upcoming Releases page, and the link target does not exist in one version of the docs, the docs engine exits with an error during the build.