Add CHANGELOG as vendored file

[olleolleolle]

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

The problem was that a link to the CHANGELOG was 404ing. So, (in a now-closed PR) I made the link absolute, instead of relative, to work around the problem.

But, that'd take us to another website to look at Bundler's CHANGELOG. It'd be nice if Bundler's website knew about the CHANGELOG - had it locally.

What was your diagnosis of the problem?

My diagnosis was we didn't have all files that would be necessary to create a local CHANGELOG, we'd have to point outside this repo, to some on-GitHub data, not very nice. Not rendered in this nice way.

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

Include CHANGELOG as a file in the generated site.

This fixes #1233.
This fixes #846

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

I chose this fix because we mentioned it in #1233.

[deivid-rodriguez]

I built a review app for this and the layout does not look good. I think you need to configure it here:

bundler-site/config.rb

Line 190 in 9cbd24f

page "/conduct.html", layout: :two_column_layout

.

Also probably make it searchable here:

bundler-site/config.rb

Line 10 in 9cbd24f

search.resources = ["index.html", "guides/", "#{config[:current_version]}/", "compatibility.html", "conduct.html", "contributors.html"]

.

[olleolleolle]

First: ✅ Now, with the two-column layout, much easier to read:

Second: ✅ https://bundler-site-add-ch-lrpv3eaqw1.herokuapp.com/doc/contributing/bug_triage.html this page now links correctly to changelog.html

❓ I was not able to use the search box and hit anything in the changelog.html file. EDIT: I checked for the word "Pledge", which is in the Code of Conduct document, and this was also not findable. Reported as a separate Issue.

[deivid-rodriguez]

Amazing @olleolleolle, thanks so much!