Extend introduction and remove/edit notes

[nicorusti]

References to drafts not meant to be published should go away, e.g.

   *Note:* For extended information on the SCION next-generation inter-
   domain architecture, see [CHUAT22], especially Chapter 2, as well as
   the IETF Internet Drafts [I-D.scion-overview] and
   [I-D.scion-components].

Same issue in other drafts:

• DP Extend introduction and remove/edit notes scion-dp_I-D#10
• CP Extend introduction, add overview and remove/edit notes scion-cp_I-D#14