rhivos-docs-convert snippet handling

[kleinffm]

Resolve markdown snippets in place. If markdown snippets must convert to asciidoc equivalents:

• prefix files with snip_
• output them to a artifacts/snippets/ directory if possible
• ensure snippet files include a :_mod-docs-content-type: SNIPPET content type declaration, contain all formatting within the file, and otherwise comply with the AsciiDocDITA Vale regex used to identify them

For example:

Output this in the snippet, so the include statement is self-contained:

:_mod-docs-content-type: SNIPPET
// MODULE: snip_tech-preview.adoc
// SOURCE: tech-preview.md

[IMPORTANT}
====
Technology Preview boilerplate text...
====

Not this, where the formatting surrounds the include statement in the module:

[IMPORTANT]
====
\include::snip_tech-preview.adoc
====