Text formatting conventions: blockquote style for non-normative text, avoid Introduction headings

[zimeon]

Discussion and desision on editors' call:

1. Adopt the blockquote style for non-normative portions of text and avoid non-normative section marking except for the Introduction (following Use blockquote for non-normative content (except intro) #171)
2. Avoid "Introduction" sub-sections and instead have a block of pre-amble text (perhaps non-normative) under enclosing section heading. This means that, for example, that the "6.1 Introduction" heading should be removed.