Document Links are user-defined (like fields, templates, etc) so there are no constant values for them.  You can use LFDatabase.GetAllLinkTypes() to return a collection of LFLinkType objects.  That class exposes properties for the ID and names.  You can also refer to the link_types table to see the current list of link types.

That said, the Supersedes/Superseded by and Attachment/Message link types do exist by default and will probably have ids 1 and 2, respectively.  This is just because they were the first 2 created, not because those ids were reserved for them.  If you want to make your code portable across repositories, you should avoid hard-coding the ids.