ENH: Allow admonitions to be style as a sidebar

[drammock]

This makes it easy to style docutils .. sidebar:: to match our admonition style, by adding classes like this:

.. sidebar:: Some title
   :class: admonition warning

   Some content

[drammock]

here's the demo:
https://pydata-sphinx-theme--836.org.readthedocs.build/en/836/demo/theme-elements.html#admonition-like-sidebars

Here's the original (unaffected) sidebar from kitchen-sink:
https://pydata-sphinx-theme--836.org.readthedocs.build/en/836/demo/kitchen-sink/paragraph-markup.html#sidebar

[choldgraf]

This looks pretty good to me! Two quick questions:

1. Given that the sidebar is a bit more limited in space, I wonder if it'd be better to not apply the left margin to the content. E.g. here's what it looks like with that removed:

   

2. Do you think this should just be the default styling for the sidebar? Maybe it would be a grey color if no :admonition: is given, and it could be colored by adding note etc?

3. Alternatively, we could keep the sidebar as-is, and add a helper class for admonitions that would make it behave like a sidebar. E.g. we could ask people to do:

   ```{note}
   :class: sidebar
   My note
   ```

[drammock]

not apply the left margin to the content

Good idea

3. keep the sidebar as-is, and add a helper class for admonitions that would make it behave like a sidebar.

No strong feeling here. Both seem equally usable. Only difference I can think of is the actual HTML (whether it's an aside or not), which makes me lean slightly toward the current implementation.

[drammock]

Do you think this should just be the default styling for the sidebar?

I gave this some thought, and I think no. We don't know what users will do with the sidebar - maybe it's just an image, we should leave this container flexible by default. They should be able to get the grey one via :class: admonition (without also specifying a semantic color)

[choldgraf]

I think the biggest benefit of going the opposite direction and supporting:

```{note}
:class: sidebar
foo
```

would be on the implementation side. E.g. if we just add these 4 CSS rules to our admonition (e.g. triggered via .admonition.sidebar, then we get pretty much the same effect:

{
    max-width: 40%;
    float: right;
    clear: both;
    margin-left: 0.5rem;
}

results in:

[drammock]

Ok I'm convinced. Feel free to push that change, I'm out of office until Tuesday.

[drammock]

Although I think you also need a rule for the inner content left margin. But still, that's a much nicer implementation

[choldgraf]

Sounds good, I will try and give it a shot if I have a moment. I am hoping to do a little flurry of PR merges so that we can make a release relatively soon.

[choldgraf]

OK, the latest pushes re-work this to trigger the behavior with a sidebar class on an admonition, rather than an admonition class on a sidebar. I also updated the docs accordingly!

I will merge this once the RTD docs pass and I re-confirm that it looks as-expected, since @drammock said he's out for several days :-)

[choldgraf]

Ah one extra change here - I realized that we were starting to put "instructional" information in the theme-elements.md file. In addition, a few of the other "theme-specific styles" pages in the demo/ folder were also acting more like instructions for how to set up various other extensions with the theme. So I decided to move these into the user_guide in a `"content and features" section, since I think that's a natural place for people to look to find out how to do specific things with this theme