chore: (mkdocs) upgrade mkdocs and material to latest versions

[chris-bateman]

Updated mkdocs and material to the latest version.
Used this guide - https://squidfunk.github.io/mkdocs-material/upgrade/

Updated overrides and added nav item to get the menu to work again.

The nav menu is slightly different compared to what is currently deployed but is functionally identical.

Builds correctly locally and pages load.

[chris-bateman]

Covers part of #151

[nphuhuy]

thanks

[pylixm]

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