docs: state variable

[hollandjg]

Description

Add really basic introductions to the State and Variable objects, and their fundamental properties.

This is meant to be complementary to the cycle tutorials which go into more in-depth examples.

If the issue is on Github, simply link to it using the '#' symbol; otherwise, provide a notion page link):

• resolves Add Documentation on State Logic (or make sure it already exists) #65

Type of change

• docs: Documentation only changes

Features (Optional)

• Just added documentation pages

Questions (Optional)

• Are these understandable?

[benwandrew]

tests are taking a while, but looks good to me!