Create user groups for hiero-mirror-node-explorer

[hendrikebbers]

As defined in https://github.com/hiero-ledger/governance, we need to create two groups for each project: A group that contains 2-3 maintainers of the project and a group with all project committers. The groups can be added to https://github.com/hiero-ledger/governance/blob/main/config.yaml by doing a PR.

[SimiHunjan]

This should be @svienot and @ericleponner

[svienot]

Hi @hendrikebbers -- any chance we can simply pick the hiero-explorer repository/project name rather than hiero-mirror-node-explorer (kind of heavy...)? Once we confirm this name, I'll take care of the PR. Thanks.