Add {embed} and {include} roles for inserting labeled content inline.

[tylere]

It would be useful to be able to insert content inline, instead of getting the nested <div> formatting generated by the directives.

For example, here is what the {embed} directive produces:

<div data-name="output-decoration-notebook-output">
  <div data-name="outputs-container" data-mdast-node-id="PJTgFejfcgMcnWeVp1RUz" class="max-w-full overflow-y-visible overflow-x-auto m-0 group not-prose relative text-left mb-5">
    <div data-name="safe-output-stream">
      <div>
        <pre class="myst-jp-stream-output text-sm font-thin font-system">
          <code>
            <span>I just want to insert this content!</span>
          </code>
        </pre>
      </div>
    </div>
  </div>
</div>

[tylere]

Opened issue #2646 to address this for {embed}.