Update documentation to recommend version pinning

[ianfixes]

Issue Summary

This project follows the SemVer standard of versioning its releases, to help users make informed choices about updating their dependencies on the arduino_ci gem.

However, all the documentation provides examples that explicitly install the latest version. This has led to new changes actively breaking other users' builds.

We should fix the documentation to show best practices: version pinning by default.

Possibly, also write a script to ease the update process (automatically bump the version, commit it, push a feature branch, and open a PR) for users who have a lot of repositories -- @RobTillaart for example.