7: Add letter header CSS to letter header section

[vr8hub]

In the discussion about letter footers, we specify the CSS that is to be used. We currently don't do this with the letter headers, and we should explicitly say how headers are to be formatted, so this adds it.

[acabal]

I don't think that's always true? Letters vary so widely in style and I don' think it's uncommon for some headers/datelines to be on the left. Forcing a right default would confuse people in the case where for whatever reason the letter header actually has left alignment.

On the contrary, letter footers are indeed (almost?) universally right-aligned.

[vr8hub]

We right-align the footer even if it's not in the source (they are indeed not universally right-aligned, I come across left-aligned ones on a regular basis). And, when the header is right-aligned, the producer has to add CSS to keep the salutation from being right-aligned, which "confuses" people, but your response has always been (and I agree) that we can't cover every possibility in the manual. Our default for letter headers is to right-align them; we should say that, otherwise, people don't know to do it. (Which is what just happened, again, in a review.) If you want to put text there instead of default CSS, great, but we should do one or the other.

[acabal]

I would prefer to have it as a written directive and not default CSS. Having it as CSS invites copy and paste in a situation where people instead need to think about the actual formatting they're trying to achieve.