Build docs action

[jbourassa]

In #375, I tried re-enabling doc generation on releases by calling the "publish doc" action in the release workflow. That didn't work: the "publish doc" action requires doc to be built beforehand in the CI workflow, but the CI workflow does not run on release.

I'm fixing it by moving the documentation generation from the CI workflow to its own workflow. The new doc workflow runs on push and commit, just like CI. It's desirable that doc generation runs on any commit to catch issues, e.g. this script that ensures documented constants and methods indeed exist:

wasmtime-rb/rakelib/doc.rake

Lines 12 to 45 in ae2e78d

	t.after = -> do
	$LOAD_PATH.unshift File.expand_path("../../lib", __FILE__)
	require "wasmtime"
	errors = []
	YARD::Registry.each do |yard_object|
	case yard_object.type
	when :module
	mod = Object.const_get(yard_object.path)
	errors << "Not a module: #{mod}" unless mod.is_a?(::Module)
	when :class
	klass = Object.const_get(yard_object.path)
	errors << "Not a class: #{klass}" unless klass.is_a?(::Class)
	when :method
	namespace = Object.const_get(yard_object.namespace.path)
	case yard_object.scope
	when :class
	namespace.singleton_method(yard_object.name)
	when :instance
	namespace.instance_method(yard_object.name.to_s)
	else
	# Unknown scope, we should improve this script
	errors << "unknown method scope '#{yard_object.scope}' for #{yard_object.path}"
	end
	end
	rescue NameError => e
	errors << "Documented `#{yard_object.path}` not found: \n #{e.message.split("\n").first}"
	end
	if errors.any?
	errors.each { |error| log.warn(error) }
	exit 1
	end

I tested the workflow by pushing new tags in a fork; it should work this time.

Fixes #377

[jbourassa]

Unrelated to the issue but that shaved off a few minutes.

[jbourassa]

Also unrelated, but that sped up things and I can't see a reason not to use it. It's set to true elsewhere.