[ENH][Documentation][Schema] Add Template Schema for BEPs

[bendhouseart]

It would be useful to have a skeleton or a template of the schema portion of a BEP. Something similar to the books dataset on datalad. This documentation would live on bids-extensions.readthedocs.io and be referenced onbids.neuroimaging.io as a part of the Get Involved section with respect to BEPs.

Documentation should follow template schema such that a user is able to:

1. Render the template into the RTD site
2. Be shown how to run tests, template should pass tests at the start
3. Template should be included in the schema? This would help to ensure that it remained up to date, but we don't really want to render it as it's a template.
4. Be used with bids-schema-tools as well, why not?

I'm also open to using a settled modality/examples already in the spec for illustrative purposes.