[docs][material-ui] Improved documentation for indeterminateIcon prop

[marctaylor01]

I have added more detailed documentation about how do customize the indeterminate icon for the
checkbox component.

Fixes #43499

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

[mui-bot]

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• docs/data/material/components/checkboxes/checkboxes.md

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[marctaylor01]

Hey @aarongarciah !
I'm very new to open source and was wondering what I should do about my pull request. On the first PR the "ci/circleci: checkout" test did not pass, so I ran pnpm deduplicate according to the CONTRIBUTE.md file. The test passed but the pnpm-lock.yaml file now has merge conflicts which I do not really understand, could you help me into correct direction of what to do next?

Thanks in advance! :)

[aarongarciah]

Hey @marctaylor01! Thanks for opening the PR. I'll review it next week, but you'll need to undo changes in the pnpm-lock.yaml file and rebase this PR on top of master.

[marctaylor01]

Hey @aarongarciah!
I've done the changes as mentioned and was wondering how it's looking with the PR review?

[aarongarciah]

Thanks for working on this! Left a couple of comments.

[aarongarciah]

Thank you @marctaylor01!