Documentation of Release Model

[SimonBiggs]

Provide details of how each version is handled, and specifically what is meant by the version number, and what parts of PyMedPhys are covered by backwards compatibility.

Lean on the following resources:

• https://pandas.pydata.org/pandas-docs/version/1.0.0/development/policies.html#policies-version
• https://discuss.streamlit.io/t/adding-beta-and-experimental-channels-to-streamlit/2765