The name "endpoint" is overloaded and we decided a while ago to not
use it for things other than `iroh_net::Endpoint`.  This further
renames more places that were called endpoint with the new name we
already introduced for them: direct address.  The direct address
naming is already used consistently on public APIs, this is only for
internal consistency.

Some more re-namings and docs tweaks

Co-authored-by: Divma <26765164+divagant-martian@users.noreply.github.com>
Co-authored-by: Philipp Krüger <philipp.krueger1@gmail.com>