SMIRNOFF spec should link to stable versions of external docs

[mattwthompson]

@jchodera noted that many links to (at least) OpenMM docs point to moving targets, things like /development/ that are automatically updated whenever a new version comes out. He suggested, I think to general agreement, that these links should be updated to point to specific versions, which won't change when a new version is released.

This shouldn't be a massive undertaking, but would require some care and attention to complete.