DOC: Revise guidance on file format selection and introduce downstream-oriented principles

[oesteban]

This PR revises the “Generic guidelines” section of the BEP Guidelines to clarify and expand the community's position on selecting file formats.

The original document presented a weak and somewhat outdated recommendation for format choices, including the bvec/bval format as a positive example, despite its known limitations. This PR removes that example and introduces a new subsection: “Choosing file formats: downstream- vs upstream-looking”.

Key changes:

• Introduces the concept of downstream-looking formats as the recommended default for both raw and derivative data.
• Clearly distinguishes upstream-looking formats that originate from data acquisition devices and should be used with caution.
• Emphasizes the importance of selecting formats that are interoperable, scalable, and familiar to researchers beyond a specific modality.
• Makes the case that downstream-oriented formats should not be limited to derivatives—favoring them in raw data improves the viability of generating robust BIDS-Derivatives.
• Notes that historical prevalence is not sufficient justification for format inclusion.

These revisions aim to make BIDS more usable, accessible, and sustainable by encouraging choices aligned with open data science ecosystems and modern computational workflows.

No changes to the BIDS specification or schema are included in this PR; it solely updates guidance for BEP authors.