💅 🖼️ Add topic directive

[agoose77]

This PR adds the RST topic directive, which behaves like an aside, with a stylised heading.

Here's an example RST:

.. topic:: Bees Talk About Work Life Balance
  :class: 

   Bzzbzz bzz bzzbzz bzz bzzbzzbzzzzz!!!?? Bzz.

This generates the following HTML in Sphinx/docutils:

<aside class="topic some-class">
  <p class="topic-title">About Bees</p>
  <p>Hello bees!</p>
</aside>

Our MDAST with this PR is

{
  "type": "aside",
  "kind": "topic",
  "children": [
    {
      "type": "paragraph",
      "children": [
        {
          "type": "text",
          "value": "Bees Talk About Work Life Balance"
        }
      ]
    },
    {
      "type": "paragraph",
      "children": [
        {
          "type": "text",
          "value": "Bzzbzz bzz bzzbzz bzz bzzbzzbzzzzz!!!?? Bzz."
        }
      ]
    }
  ],
  "class": "some-class"
}

I welcome any suggested changes here.

One extraneous change is the addition class and label (identifier) attributes to the aside AST node type. I think it's our goal to add this to all MyST nodes, right? Perhaps that would be done on the base Node type, rather than per-node like we currently do in myst-spec?

[rowanc1]

In sphinx this looks like:

I think we should handle the header properly. I think that this should eventually be title as a node type, and be shared between admonitions, drop-downs, topics, exercises, etc. But for this PR we should adopt admonitionTitle and then follow up with fixing #1139.

As another bug, we are dropping other aside titles.

[agoose77]

This PR avoids the new node type by assuming that the first child is the title. I don't hate that, but I recognise that we long-term want #1139

[rowanc1]

Looking through this, I think we just want to merge with aside and expand to supporting optional titles in the others. Right now that is a bug, and then expanding that, both are the same anyways.

https://jupyterbook.org/en/stable/content/layout.html#sidebars-within-content

[rowanc1]

Making it look like this:

I think there are some improvements to be made, but I think this is probably good enough for a first cut?

[rowanc1]

Going to move ahead with this for now to queue up a release - we can always come back and make improvements.