Add stacked ensembling to AutoML documentation

[angela97lin]

Closes #1329

Docs here

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Codecov Report

Merging #1425 (816d5e4) into main (e2b1030) will not change coverage.
The diff coverage is n/a.

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[jeremyliweishih]

LGTM just some comments!

[freddyaboulton]

@angela97lin This looks good to me! This is out of scope of this PR but this sentence might be confusing to users because I don't think we ever explain how the batching in AutoML works:

The stacking ensemble pipeline runs in its own batch after a whole cycle of training has occurred (each allowed pipeline trains for one batch). Note that this means a large number of iterations may need to run before the stacking ensemble runs.

One thing we can do is add something to the AutoML printout like this if the number of iterations selected by the user is too small:

Ensembling set to true but the number of iterations is too small
for ensembling to run. Set it to at least ...

and

Ensembling is set to True and max_iterations is set to ... 
so ensembling will run in batches 4, 8, 12 

That way users are not so confused about whether or not ensembling will run. Like I said, out of scope of this PR, but I think this would be a useful enhancement in the future!

[bchen1116]

LGTM! Just left one comment but not blocking

[dsherry]

@angela97lin looks good to me!

I only had one change request, which was to use max_batches=5 (or 4? forgetting) instead of max_iterations=20

[angela97lin]

@freddyaboulton Great suggestion!! I filed #1461 to track this 😁