Author spec.bs with metadata block and boilerplate omits

[GarthDB]

Part of #1006.

Create the top-level spec.bs with Bikeshed metadata block:

<pre class='metadata'>
Title: Spectrum Design Data Specification (Bikeshed Prototype)
Shortname: spectrum-design-data
Level: 1
Status: LD
Group: Adobe Spectrum
URL: https://garthdb.github.io/design-data-spec/
Editor: Garth Braithwaite, Adobe https://www.adobe.com, garthdb@adobe.com
Abstract: Prototype rendering of the Spectrum Design Data spec using Bikeshed.
Markup Shorthands: markdown yes, css no
Boilerplate: omit conformance, omit copyright, omit feedback-header
</pre>

Status: LD (Living Document) avoids W3C TR boilerplate. The omit directives strip W3C-process scaffolding that doesn't apply to an Adobe vendor spec.