The documentation is outdated

The purpose of this bug report to group track different outdated items related to documentation.

The list of known issues:

1. rally.readthedocs.io includes documentation for both rally framework and rally-openstack that requires to add a lot of notes that specific package should be installed

2. installation page includes reference to install_rally.sh script that was deprecated and removed long time ago. (solution for those who bumped into the issue: use `pip install rally` or `pip install rally-openstack`)

3. installation page and hub.docker.com include duplicated info about docker

4. There is no reference to Task input format v2 at all.

5. All examples of how to develop plugins are openstack related and use wrong imports (after splitting openstack plugins into the separate framework)

6. Changelog is not included into documentation