Clarify security story - take 2 (based on PR #378)

[caridy]

Changes:

• new section about security with all the details
• explanation based on the taxonomy used for this proposal
• removed the concept of "trusted" and "untrusted" from the explainer.
• ShadowRealms -> ShadowRealm

[caridy]

@leobalter can you take a look at this, it is pending to be merged.

[leobalter]

LGTM w/ +1 to @erights suggestions.

I just wanna give it another look in the morning time before merging it.