this makes sure the portal div is there for both client and server
so we don't get any hydration issues. this changes the Popover component
to render the portal conditionally so the DOM matches at first

`Button` can be used with the `as` prop,
which makes `variant` not make much sense

Update schema to have the type Mdx contain an mdxAST field to maintain feature parity with Gatsby v4. Add remark and remark-mdx to accomplish this, but needed older versions to use require syntax

when `externals` is an object here, it overwrite's Gatsby's `externals`
config, which causes all sorts of errors with the different modules that
their `externals` defines, like `util` and `stream`.
making our `externals` an array appends them to Gatsby's,
instead of replacing theirs

previously this was a dependency of Gatsby, but no longer

there are slightly different classnames bc of the emotion upgrade

as part of the upgrade to Gatsby 5, the `css` prop wasn't working on
styled components or regular JSX tags, but _only_ in templates.
regular pages are fine.
i spent days trying to figure out wtf was going on and wasn't able to

Liz figured out that switching from the `css` prop to `styled` works,
so instead of toiling away at a heap of config files, we can just do this

code blocks are now rendered as `pre` tags from MDX,
and inline code tags are rendered as `code` tags

the anchors in headers were offsetting headers to the right.
they weren't getting the styles they should have.
i think this might have to do with the emotion issue.
this fixes the issue by adding the expected styles with
a custom class name

Upgrade Gatsby v5