Extend introduction, add overview and remove/edit notes

[elear]

*Note:* It is assumed that readers of this draft are familiar with
the basic concepts of the SCION next-generation inter-domain network
architecture. If not, please find more detailed information in the
IETF Internet Drafts [I-D.scion-overview], [I-D.scion-components],
[I-D.scion-dp], and [I-D.scion-cppki], as well as in [CHUAT22],
especially Chapter 2. A short description of the SCION basic terms
and elements can be found in Section 1.1 below.

the first two documents are not in my queue and I don't expect them.

Same issue in other drafts:

• DP Extend introduction and remove/edit notes scion-dp_I-D#10
• CPPKI Extend introduction and remove/edit notes scion-cppki_I-D#29

[nicorusti]

• on the ISD definition, change language on jurisdiction (i.e. --> e.g.)