Add yaml spec generation

[2maz]

The spec file is intended to be used for validation of the table spec implementation, e.g., as in slurm-monitor's sqlalchemy definitions

Example output

# AUTOMATICALLY GENERATED BY `process-doc -w -yaml`.  DO NOT EDIT.
# Edit `util/formats/newfmt/types.go`, then in `util/process-doc` run `make install`.

MetadataObject:
  fields:
    producer:
      type: 'NonemptyString'
      doc: 'The name of the component that generated the data (eg "sonar", "slurm-monitor")'
    version:
      type: 'NonemptyString'
      doc: 'The semver of the producer'
    format:
      type: 'uint64'
      doc: 'The data format version'
    attrs:
      type: '[]KVPair'
      doc: 'An array of generator-dependent attribute values'
    token:
      type: 'string'
      doc: 'EXPERIMENTAL / UNDERSPECIFIED.  An API token to be used with Envelope.Data.Attributes.Cluster, it proves that the producer of the datum was authorized to produce data for that cluster name.'
``

[lars-t-hansen]

LGTM. Before landing, please run make install here so that all output files are in sync with the script, and add the generated files to the commit. Also please squash changes when landing.