ENH: Introduce decision flowchart and policy for new data types, modality suffixes, and formats in BEPs

[oesteban]

This PR introduces a structured decision-making framework to guide the evaluation of BIDS Extension Proposals (BEPs) that seek to introduce:

• New data types (e.g., top-level folders like micr/, beh/)
• New modality suffixes (e.g., _xyz)
• New file formats (e.g., .h5, .mat)

It provides a mermaid flowchart and accompanying criteria to:

• Clarify when a BEP is required
• Define review thresholds for foundational additions
• Ensure early, consistent, and transparent evaluation across proposals

This addition responds to recent discussions at the BIDS meeting currently being held in Copenhagen, where multiple BEPs introducing new modalities are under simultaneous review.

While @neuromechanist’s proposal in #2135 focuses on extending specific modality definitions, my position is that BIDS governance currently lacks a clear, principled process for assessing such proposals. Without that, we risk introducing inconsistencies or overly flexible definitions directly into the specification without first resolving structural questions at the framework level.

This PR aims to equip the BIDS community with tools to make better decisions.

X-References: bids-standard/bids-specification#2108
X-References: bids-standard/bids-specification#2135