Move upgrade instructions to a separate page

Currently upgrade instructions are listed in `Changelog` but it isn't right because of the following facts:
- Upgrade instructions in most cases should upgrade from `feature` versions and not `bugfix` versions. We have currently 6 upgrade instructions: `from Titan to JanusGraph 0.1.x`, `from 0.1.x to 0.2.x`, `from 0.2.0 to 0.2.1` (exceptional case), `from 0.2.x to 0.3.x`, `from 0.3.x to 0.4.x`, `from 0.4.x to 0.5.x`.
- Currently upgrade instructions are hard to be find because you should go to `Changelog` and find the fist version of a specific feature version to which you would like to upgrade. It isn't intuitively as for me.

I would like to propose to move all upgrade instructions to a separate page.

Start by reviewing the upgrade instructions currently listed in Changelog, including the six version transitions named in the issue. Move those instructions to a separate page and verify that the complete set remains available there in an easier-to-find structure.

Written by the indexing model from the issue text.