Note on best practices for overriding DCC CSS

[wbrgss]

Add a note explaining CSS specificity and the best way to override Dash-included CSS (e.g. from DCC). I think the best recommendation we could give is suggest users create a global container with a className (or an id) and prefix the selectors they want overridden with this identifier. We could recommend this over trying to use !important or trying to change the loading order of the CSS.

Prompted by https://www.omnisci.com/blog/plotly-dash-and-omniscidb-for-real-time-data-visualization (see "Controlling the Loading Order of Dash CSS")