Move install and upgrade instructions into the main Tracks release

This is my suggestion for moving the manual into the releases as mentioned in #11.

This pull request accompanies TracksApp/tracks#140.

The links in this pull request won't work until it is merged alongside a 2.2 release, but the accompanying installation and Upgrading docs in the main tree will look like this:

[Installation](https://github.com/TracksApp/tracks/blob/move-manual/doc/installation.textile)
[Upgrading](https://github.com/TracksApp/tracks/blob/move-manual/doc/upgrading.textile)