Document that symlinks in a content repository are supported and how to use them

Document that, starting in Antora 3, symlinks in both the git tree of a content repository and in the worktree(s) of a content repository are supported. This includes both file and directory symlinks.

This documentation is really to say, yes, symlinks are supported. But it should go on to point out any limitations (like the fact that a symlink in a git tree cannot point to a file outside the repository and that a symlink cannot be cyclic).

The documentation should present symlinks as a solution for organizing files into an Antora structure without having to move them. It should also present them as a way to map code from a test suite into the Antora structure so that it can be included into source blocks.

Taking one of these use cases, the documentation should show how to create and commit a symlink

The documentation should clarify that Antora treats the symlink as though it is a concrete file, and only follows the symlink to get the contents and stat. In other words, the output of a symlink is a concrete file, not a symlink.

The documentation should also point out that, since Windows 10, symlinks are supported on Windows. Reference: https://blogs.windows.com/windowsdeveloper/2016/12/02/symlinks-windows-10