Migrate docs for deprecated React components

[camertron]

Deprecated component status

This PR migrates docs for deprecated React components from the old primer.style/react site to the new docsite. This was not a straightforward migration, since the current docs exist within this matrix of sadness:

Component	Has non-dep counterpart	Bespoke .md file	components.json?	Type of props table
ActionList	✅	✅	🚫	Hand-written md
ActionMenu	✅	✅	🚫	Hand-written md
Button	✅	✅	🚫	Hand-written React
FilterList	🚫	✅	✅	Imports .data.json
FilteredSearch	🚫	✅	✅	Imports .data.json
SideNav	🚫	✅	🚫	Hand-written md
UnderlineNav	✅	✅	🚫	Imports .data.json

Legend:

• Has non-dep counterpart: The deprecated component has a newer, non-deprecated version.
• Bespoke .md file: At least some of the docs for the deprecated version of the component live in a separate, individually-maintained .md file that contains additional information to what may be available in components.json.
• components.json?: The deprecated component is listed in components.json, with a status of Deprecated.
• Type of props table: How the props table is written in the bespoke .md file. "Hand-written React" means it uses <PropsTable> component. "Imports .data.json" means it imports the corresponding component_name.data.json file and passes the data to the <ComponentProps> component.

Action taken

1. React components can now have multiple versions. Each version can be reached by way of a dropdown menu that appears on each component page. The URLs for these components take the form /components/component-name>/react/status, where the status is eg. "alpha," "beta," "draft", "deprecated," etc. This follows the same URL convention as Rails components. A special URL, /components/component-name/react/latest, redirects to the most stable version of the component, usually "alpha."
2. For ActionList, ActionMenu, Button, and UnderlineNav, I copy/pasted the existing docs into dedicated pages under components/component-name/react/deprecated, which follows the same naming conventions I described above. I added a special reactStatus: deprecated field to the page's YAML frontmatter to mark components that have dedicated pages for their deprecated versions, which gatsby-node.js queries when generating pages.
3. Since FilterList and FilteredSearch are listed in components.json, I elected to turn them into top-level /deprecated-components, i.e. under the "Deprecated components" section in the nav. Although there is some additional information in their bespoke .md files, it's minimal enough to drop from the new docsite.
4. SideNav now lives under the top-level "Deprecated components" nav section. There was enough information in its bespoke .md file that I decided to port it over by creating /deprecated-components/side-nav/react/latest.md, which is consistent with our now site-wide URL structure.

Prop tables

1. For hand-written markdown, I copied the props table over without modification.
2. For hand-written React, I copied the props table over without modification. I also copied over the <PropsTable> component from Primer React.
3. The contents of the .data.json files used by FilterList FilteredSearch are also part of components.json. By opting to use react-component-layout.tsx for these components, the props table is now generated automatically. The results should appear identical.

Additional hurdles

Because the new docsite uses an outdated version of Gatsby, we are unable to use modern versions of Primer React to build it. As a direct consequence, I had to re-implement the old docsite's ThemeReferenceTree component, which relies on Primer React's TreeView component. Sadly TreeView isn't available in older versions of the library. I had to rewrite it to use <summary>/<details> elements instead.

[keithamus]