Missing documentation for autocorrelation

### Description

Autocorrelation describes the correlation of data with itself but shifted. It helps identifying randomness or time dependent patterns. The autocorrelation in `Correlation` module lacks in proper [documentation](https://fslab.org/FSharp.Stats/Correlation.html#Autocorrelation) and should be verified.

The example in https://www.itl.nist.gov/div898/handbook/eda/section3/eda35c.htm cannot be reproduced with the current implementation.

To get a visual impression what the implemented autocorrelation function does, run the following. It shows a gaussian distribution that is shifted with different lags.

```fsharp
#r "nuget: FSharp.Stats, 0.5.0"
#r "nuget: Plotly.NET"

open FSharp.Stats
open Plotly.NET

let x = [0. .. 100.]
let gaussPDF = Distributions.Continuous.Normal.PDF (10.) 2.
let yGauss = x |> List.map (gaussPDF) |> vector

let plotCorr lag =
let v1' = yGauss.[0..(yGauss.Length-1 - lag)]
let v2' = yGauss.[lag..]
[
[
Chart.Point(v1'|> Seq.indexed,Name="v1")
Chart.Point(v2'|> Seq.indexed,Name="v2")
]
|> Chart.combine
|> Chart.withXAxisStyle "x values"
|> Chart.withYAxisStyle "y values"

Chart.Point(v1',v2',Name="correlation") |> Chart.withXAxisStyle "v1 values" |> Chart.withYAxisStyle "v2 values"
]
|> Chart.Grid(2,1)
|> Chart.withTitle (sprintf "pearsons r: %.2f" (Correlation.Seq.pearson v1' v2'))
|> Chart.withTemplate ChartTemplates.lightMirrored
|> Chart.show

plotCorr 0
plotCorr 1
plotCorr 80
```


### Pointers

- Check out the current [documentation](https://fslab.org/FSharp.Stats/Correlation.html#Autocorrelation).
- Extend the description of the documentation
- use cases of autocorrelation
- special cases
- more examples
- Should negative lags be a valid input? If yes, is it possible or has this functionality to be added?
- Of course you can start writing in e.g. markdown/this issue and afterwards we try to incorporate into the library.

### References

Hints (click to expand if you need additional pointers)

- To be able to contribute to this library you'll need
- an GitHub account
- an IDE like Visual Studio Community or Visual Studio Code
- [dotnet 6 sdk](https://dotnet.microsoft.com/en-us/download)
- to build the binaries yourself follow the [instructions](https://fslab.org/FSharp.Stats/#Installation)
- while working on the [FSharp.Stats documentation](https://fslab.org/FSharp.Stats/) (any file within https://github.com/fslaborg/FSharp.Stats/tree/developer/docs) you can navigate to the project folder with a prompt of your choice and use the command `./build watchdocs`
- unit tests can be executed via `./build runtests`