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 https://github.com/scionassociation/scion-dp_I-D/issues/10
• CPPKI https://github.com/scionassociation/scion-cppki_I-D/issues/29

[nicorusti]

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