Revise docs for v25 features to add `versionadded` directives

[genehack]

These availability statements are great, but it makes me realize we should be using

.. versionadded:: x.y.z

directives in our Sphinx/rST docs so those availability statements are also there.

_Originally posted by @tsibley in https://github.com/nextstrain/nextstrain.org/pull/1004#discussion_r1735082881_