Add missing /whats_new.html page

[deivid-rodriguez]

What was the end-user problem that led to this PR?

The problem was the getting started page includes a link to Bundler 2.4 what's new page.

Instead, it should link to the latest version.

What was your diagnosis of the problem?

My diagnosis was that this page started being missing when it was migrated to HAML in f2c455d, because we create proxy pages, but only for .haml pages:

bundler-site/config.rb

Lines 49 to 58 in cf57e55

	# Make documentation for the latest version available at the top level, too.
	# Any pages with names that conflict with files already at the top level will be skipped.
	Dir.glob("./source/#{config[:current_version]}/*.haml").each do |file_path|
	file_path = file_path.sub(/\.haml$/, "")
	page_path = file_path.sub(/^\.\/source\//, "")
	proxy_path = file_path["./source/#{config[:current_version]}/".length..-1]
	proxy proxy_path, page_path unless file_exist?(proxy_path)
	end

It was replaced with a hardcoded link to the v2.4 version, which was the latest at the time, but it got out of date.

What is your fix for the problem, implemented in this PR?

My fix is to create a symlink like we did in #1424.

Why did you choose this fix out of the possible options?

I chose this fix because because it's more SEO friendly that a client side redirect.

[olleolleolle]

More uniform, better! Thanks!