Fix install directions for all docs using no_trainer scripts
# What does this PR do?

This PR fixes the directions in the documentation for installing Accelerate, as pointed out here: https://github.com/huggingface/accelerate/issues/349

Currently they state to install just with pip, but as they are changed based on the git version of Accelerate, users should install with it instead.

The general Accelerate docs can remain the same however. 

## Who can review?
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
_The documentation is not available anymore as the PR was closed or merged._