WIP: Add a snippet template

[msuchane]

This pull request adds a new template for a text snippet file.

I'm basing the content of the snippet template of several existing sources:

• The existing concept module template, taking from it the comments around the heading, the file name, and such.
• The using-text-snippets.adoc guideline, to explain what a text snippet is and how to use it.
• The Admonitions section of the RH supplementary style guide, which clarifies the purposes of different admonitions that can appear in a snippet.

Why add a snippet template:

• We already have the guidelines on using them. See using-text-snippets.adoc.
• Snippet files are already in use, at least in RHEL documentation.
• The Jupiter platform introduces the :_content-type: SNIPPET attribute for identifying snippet files.
• Jupiter does place some restrictions on the elements that a snippet can include: snippets must not have numbered headings or IDs. Those requirements are consistent with the definition of snippets in mod docs. A formal template helps enforce the requirements.

I know that snippets have been a controversial and often discussed topic (https://github.com/redhat-documentation/modular-docs/issues/18, https://github.com/redhat-documentation/modular-docs/pull/123, https://github.com/redhat-documentation/modular-docs/pull/161), but given that snippets in our documentation are already a reality and that we've found a compromise in https://github.com/redhat-documentation/modular-docs/issues/18, I think that the template only formalizes what we already have.

[msuchane]

We've decided not to make this an official template alongside the four existing ones. The snippet template will only exist in newdoc as an internal file to simplify creating new snippets that comply with the technical requirements.

I'm closing this PR now.