Error when passing format args positionally to makedocs

[mortenpi]

We shouldn't accept format objects (e.g. HTML(...) or LaTeX(...)) as positional arguments to makedocs. Currently you can pass them as a positional arguments (e.g. makedocs(LaTeX(), ...)) because they are <: Plugin. They won't have any effect though, since the output format is handled by the format keyword.

We should probably not declare HTML etc. to be <: Plugin in the first place. Instead perhaps a new abstract Writer type?

We are actually currently messing up in the test/example builds:

Documenter.jl/test/examples/make.jl

Lines 142 to 158 in eae33a0

	examples_html_local_doc = makedocs(
	debug = true,
	root = examples_root,
	build = "builds/html-local",
	doctestfilters = [r"Ptr{0x[0-9]+}"],
	sitename = "Documenter example",
	pages = htmlbuild_pages,
	expandfirst = expandfirst,
	linkcheck = true,
	linkcheck_ignore = [r"(x|y).md", "z.md", r":func:.*"],
	Documenter.HTML(
	assets = ["assets/custom.css"],
	prettyurls = false,
	edit_branch = nothing,
	),
	)