Mention HTTPS bindings as mandatory

[ticapix]

What this PR changes/adds

Clarify that HTTPS bindings are mandatory
(and editorial changes to have one sentence per line, simplifying the future PRs and review)

Linked Issue(s)

International-Data-Spaces-Association/identity-in-data-spaces#11 (comment)

[arnoweiss]

I'm generally open to this amendment. The topic hasn't been discussed in the committer's round yet so I think a PR is a little early at this stage. Up until now, relying specifications had to specifically reference the binding to lift it to normativity, see this example_

Participant Agents MUST facilitate data exchange according to the HTTPS binding defined in the Dataspace Protocol.

The committers have to discuss the effect on the spec's releases as it's definitively a change with a normative effect.

[ssteinbuss]

Hi,
The introduction typically does not state requirements. It is descriptive. The normative statements and requirements, indicated by the keywords like MUST follow in the later section. I don't see a need to make this requirement in the intro. Is it not covered elsewhere already?