Correct the order of the articles in the `using` folder.

[Huang-Libo]

Current issue

As we can see in using/unreleased-features.html.md, the order of this article is 0, which is must wrong:

---
title: Using Unreleased Features
description: Instructions to use CocoaPods from a feature branch or a Work-in-progress fork
ignore: true
order: 0
thanks: "Sachin Palewar"
---

This wrong order number lead to the position of this article is too front:

Current order number of articles in using folder is here, there are two repetitive items:

article	order
getting-started.html	0
unreleased-features.html	0
using-cocoapods.html	1
pod-install-vs-update.html	1
the-podfile.html	2
troubleshooting.html	3
faq.html	4
test-specs.html	5
a-gemfile.html	6

Re-numbering

I think unreleased-features.html should move to the last position, and pod-install-vs-update.html should use number 2, so the following articles' order number should all add 1 .

article	order
getting-started.html	0
using-cocoapods.html	1
pod-install-vs-update.html	2
the-podfile.html	3
troubleshooting.html	4
faq.html	5
test-specs.html	6
a-gemfile.html	7
unreleased-features.html	8

After changed these order, I can see a better order for these articles (I run the website locally):