Rework CONTRIBUTING:BEPs section for schema-driven approach

[yarikoptic]

ATM it still talks about changing regexes etc. It should be

• prepare branch/PR against bids-specification with changes for the BEP
• follow instructions here and/or there which boil down to
  • convert adjusted schema: cd bids-specification; bst -v export --output src/schema.json (fix if/how needed)
  • use it to validate sample data <path to bids-validator install>/bids-validator-deno -s src/schema.json <path to dataset>

[yarikoptic]

FWIW setting up something like that in https://github.com/bids-standard/bids-specification/pull/1775/files#diff-340dd8fa4a5cba6b30520d676d1831fd82628971874ea46396e3ace6c6ef0f03 but with complication that I am also doing conversion of example datasets to BIDS 2.0.
Besides that step I think we might benefit from similar action generally be available there and thus testing across all example datasets. ATM bst unittests only test some selected few with internal validator, not JS one.

[effigies]

I have removed the legacy bits, have not yet added schema bits.