Add pyenv to Tools

[henrikvtcodes]

Since pyenv is used in the ISM app and ideally all future VERSO project written in python, it should be documented. This section should ideally cover what pyenv is used for, brief installation/setup instructions (link to repo instructions for more complicated setups or if the user encounters problems), the most commonly used commands, and how it is used in a project.
For how it is used in a project, specifically cover the use of a .python-version file.