Markdown syntax migration script

Script replacing include_code with code-snippet

Migration script: partials, some variables

Add variables to conversion script

draft repo-link component

Complete repo-link component

Migration script: handle github links

Draft include_svg→inline-svg (non-functional)

Currently doesn't work due to image path issues.
Also, captions and custom classes (for inlining) not implemented yet.

Conversion script: refactor & add code-page-name

Custom code-page-name component works around Markdoc limitation where
vars can't be used in `inline code` sections.

Migrate script: Handle more code includes correctly

Migration script: tabs and tabbed code samples

Child pages macro & conversion script

Adapted from 70cffa67ed

Migration script: update with some partial fixes

Migration script: callouts→admonitions

Fix auto-generation of index pages

Migration script: fix SVG migration

Migration scripting: fix code block prefixes & indentation

- Use the Redocly 0.66 feature for code block prefixes
- Update the script for converting indented code blocks to fences with
  Roman's latest fixes (now uses 4 spaces per level, for consistency)

content/@theme/markdoc/components.tsx

@@ -0,0 +1,44 @@
+import * as React from 'react';
+// @ts-ignore
+import dynamicReact from '@markdoc/markdoc/dist/react';
+import { usePageSharedData } from '@portal/hooks';
+import { Link } from '@portal/Link';
+
+export function IndexPageItems() {
+    const data = usePageSharedData('index-page-items') as any[];
+    return (
+        <div className="children-display">
+            <ul>
+              {data.map((item: any) => (
+                <li className="level-1">
+                  <Link to={item.slug}>{item.title}</Link>
+                  <p className='class="blurb child-blurb'>{item.blurb}</p>
+                </li>
+              ))}
+            </ul>
+        </div>
+    );
+}
+
+export function RepoLink(props: {
+        children: React.ReactNode;
+        path: string;
+        github_fork: string;
+        github_branch: string
+    }) {
+    const treeblob = props.path.indexOf(".") >= 0 ? "blob/" : "tree/"
+    const sep = props.github_fork[-1] == "/" ? "" : "/"
+    const href = props.github_fork+sep+treeblob+props.github_branch+"/"+props.path
+
+    return (
+        <Link to={href}>{dynamicReact(props.children, React, {})}</Link>
+    )
+}
+
+export function CodePageName(props: {
+    name: string;
+}) {
+    return (
+        <code>{props.name}</code>
+    )
+}