[AvatarGroup] allow cascade direction to be configurable

[dimitropoulos]

• I have followed (at least) the PR section of the contributing guide.

I am working with a design that utilizes AvatarGroup, but the design requires the groups to cascade above each other, rather than below. It seems reasonable to me that a prop for this should exist to configure this.

I will write tests as soon as it's agreed that this has enough value to be added. Here's what it looks like right now:

1. I read https://mui.com/guides/api/#property-naming and didn't think a prop named above or below has enough context. Maybe cascadeAbove or cascadeBelow or something?
2. There's more I'd like to do here to make the spacing between the avatars more configurable, but at least this is a start to use theme spacing rather than what it was before (hardcoded value).

Notes

This component is on react-ui-roundup if looking at other implementations is helpful:

[mui-bot]

Details of bundle changes

@material-ui/lab: parsed: +0.07% , gzip: +0.10%

Generated by 🚫 dangerJS against 9032d32

[mnajdova]

Before working on a new feature, please create an issue to make sure that this is a feature the core team wants the component to support. We can also discuss possible names for the new prop there.

With this process we make sure that there is not unused effort, regardless of whether it comes from the community or the team

[dimitropoulos]

@mnajdova no problem: #30789

[siriwatknp]

👋 The migration PR has been merged.

Please follow these steps to make sure that the contents are all updated. (Sorry for the inconvenience)

1. pull latest master from upstream to your branch
2. if your PR has changes on the *.md or demo files, you are likely to encounter conflict because all of them have been moved to the new folder.
   2.1 revert the change on those markdown files you did
   2.2 pull latest master from upstream (you should not see conflict)
   2.3 make the changes again at docs/data/material/*
3. run yarn docs:api
   • you might see the changes in docs/pages/material/api/* if your PR touches some of the components
   • it is okay if there is no changes

If you are struggle with the steps above, feel free to tag @siriwatknp

[siriwatknp]

I am closing this PR and let's wait for upvotes in #30789

I added the workaround in case anyone wants to achieve the overlap behavior.