Stop warning for mailto: links

[friggog]

Previously mailto: links got treated as a local link so triggered a warning

[mortenpi]

LGTM, although I am wondering if we should do this check already in isabsurl:

Documenter.jl/src/Utilities/Utilities.jl

Lines 547 to 553 in c024d21

	"""
	isabsurl(url)
	Checks whether `url` is an absolute URL (as opposed to a relative one).
	"""
	isabsurl(url) = occursin(ABSURL_REGEX, url)
	const ABSURL_REGEX = r"^[[:alpha:]+-.]+://"

[friggog]

I'm not certain I'd count a mailto link as an absolute url - not sure exactly what the definition is but to me it seems to complicate the semantics of the isabsurl function?

Easy change if you'd prefer to roll it in vs what is there now

[mortenpi]

That's fair, I was similarly unsure. For what it's worth, I think technically they are (e.g. IsURL considers them to be absolute), but we can leave figuring that out for another day. Thanks for the PR!