Add citation header

[Rafnuss]

Fix #2835

[Rafnuss]

@jayhesselberth , I had a first draft at a solution for this, but I don't really understand the mechanism to render the the data (

pkgdown/R/build-home-authors.R

Lines 11 to 19 in 6615322

	data <- list(
	pagetitle = tr_("Authors and Citation"),
	citations = data_citations(pkg),
	authors = unname(authors$all),
	inst = authors$inst,
	before = authors$before,
	after = authors$after,
	source = source
	)

) and how to change the html page. Would you mind pointing me to the right direction?