Docs build does not have `Statistics.mean()` in their namespace

[Navid C. Constantinou]

Describe the bug
Docs can't find mean function that is used in an example. See the most recent docs build log.

│ ```@example MAIN
│ ds = Systems.henon()
│ points = trajectory(ds, 2000; Ttr = 100)
│ 
│ λlocal = local_growth_rates(ds, points; Δt = 1)
│ 
│ λmeans = mean(λlocal; dims = 2)
│ λstds = std(λlocal; dims = 2)
│ fig = figure()
│ x, y = columns(points)
│ scatter(x, y; c=vec(λmeans), alpha = 0.5)
│ colorbar()
│ fig.tight_layout(pad=0.3); fig
│ ```
│   c.value = UndefVarError: mean not defined

I believe that adding Statistics in the docs/Project.toml will do the job.

[Navid C. Constantinou]

This is the reason that the figure in the end of the Docs/Lyapunov Exponents page does not appear.