Fixed typos / outdated links in README
* Website: [concourse-ci.org](https://concourse-ci.org)
* Documentation:
- * [Introduction](https://concourse-ci.org/introduction.html)
+ * [About](https://concourse-ci.org/about.html)
* [Setting Up](https://concourse-ci.org/install.html)
- * [Using Concourse](https://concourse-ci.org/using-concourse.html)
+ * [Using Concourse / Docs](https://concourse-ci.org/docs.html)
* Forum
* [Concourse Discourse](https://discuss.concourse-ci.org)
* Chat
