[Markdoc] `headings` and heading IDs (#7095)

* deps: markdown-remark

* wip: heading-ids function

* chore: add `@astrojs/markdoc` to external

* feat: `headings` support

* fix: allow `render` config on headings

* fix: nonexistent `userConfig`

* test: headings, toc, astro component render

* docs: README

* chore: changeset

* refactor: expose Markdoc helpers from runtime

* fix: bad named exports (commonjsssss)

* refactor: defaultNodes -> nodes

* deps: github-slugger

* fix: reset slugger cache on each render

* fix: bad astroNodes import

* docs: explain headingSlugger export

* docs: add back double stringify comment

* chore: bump to minor for internal exports change

1 files changed, 7 insertions, 8 deletions

diff --git a/packages/integrations/markdoc/README.md b/packages/integrations/markdoc/README.md
index 9a8bda3bb..e3cec5499 100644
--- a/packages/integrations/markdoc/README.md
+++ b/packages/integrations/markdoc/README.md
@@ -143,30 +143,29 @@ Use tags like this fancy "aside" to add some *flair* to your docs.
 
 #### Render Markdoc nodes / HTML elements as Astro components
 
-You may also want to map standard HTML elements like headings and paragraphs to components. For this, you can configure a custom [Markdoc node][markdoc-nodes]. This example overrides Markdoc's `heading` node to render a `Heading` component, and passes through [Markdoc's default attributes for headings](https://markdoc.dev/docs/nodes#built-in-nodes).
+You may also want to map standard HTML elements like headings and paragraphs to components. For this, you can configure a custom [Markdoc node][markdoc-nodes]. This example overrides Markdoc's `heading` node to render a `Heading` component, and passes through Astro's default heading properties to define attributes and generate heading ids / slugs:
 
 ```js
 // markdoc.config.mjs
-import { defineMarkdocConfig, Markdoc } from '@astrojs/markdoc/config';
+import { defineMarkdocConfig, nodes } from '@astrojs/markdoc/config';
 import Heading from './src/components/Heading.astro';
 
 export default defineMarkdocConfig({
   nodes: {
     heading: {
       render: Heading,
-      attributes: Markdoc.nodes.heading.attributes,
+      ...nodes.heading,
     },
   },
 })
 ```
 
-Now, all Markdown headings will render with the `Heading.astro` component, and pass these `attributes` as component props. For headings, Markdoc provides a `level` attribute containing the numeric heading level.
+All Markdown headings will render the `Heading.astro` component and pass `attributes` as component props. For headings, Astro provides the following attributes by default:
 
-This example uses a level 3 heading, automatically passing `level: 3` as the component prop:
+- `level: number` The heading level 1 - 6
+- `id: string` An `id` generated from the heading's text contents. This corresponds to the `slug` generated by the [content `render()` function](https://docs.astro.build/en/guides/content-collections/#rendering-content-to-html).
 
-```md
-### I'm a level 3 heading!
-```
+For example, the heading `### Level 3 heading!` will pass `level: 3` and `id: 'level-3-heading'` as component props.
 
 📚 [Find all of Markdoc's built-in nodes and node attributes on their documentation.](https://markdoc.dev/docs/nodes#built-in-nodes)