Governance Updates after AMReX Adoption

[ax3l]

For the start, we aim to have the same governance models in WarpX and AMReX.

After AMReX-Codes/amrex#3917 is reviewed and merged in AMReX, I would like to:

• copy over to WarpX the wording improvements
• copy over to WarpX the governance change in the TC how to add Contributors from a single-person decision to:

Individual TC members can suggest the addition of new contributors.
The suggestion must be seconded by another TC member.
Any TC member has the right to oppose the suggestion and call for a vote on the decision.

cc @ECP-WarpX/warpx-steering-committee (for approval)
cc @ECP-WarpX/warpx-technical-committee (for feedback)