docs: add MongoDB to users.md

[sakib]

add MongoDB as a known user of Argo Workflows

Motivation

Modifications

Verification

Documentation

[isubasinghe]

Are you part of MongoDB? Just a bit wary of merging this in without confirmation from MongoDB themselves.

[github-actions[bot]]

This PR has been automatically marked as stale because it has not had recent activity and needs further changes. It will be closed if no further activity occurs.

[github-actions[bot]]

This PR has been closed due to inactivity and lack of changes. If you would like to still work on this PR, please address the review comments and re-open.