Add Cypress tests

[Kent55]

Currently, the sidebar is not visible on the docs however, there is no test in place to detect a failure like this currently. Therefore, adding Cypress to cover e2e tests to minimise regressions making production will be beneficial.

[Kent55]

Changing priority to low as we can manage this manually by thoroughly testing each PR (until I can get round to implement Cypress).

[Kent55]

@benspring what do you reckon re the prioritisation of this vs the automation script?