Remove hubdown and use our own markdown pipeline (#17551)

* Uninstall hubdown, install unified deps

* Create create-processor

* Use it

* Update a doc

* Tweak the language

* Use the same versions that hubdown used

lib/render-content/create-processor.js

@@ -0,0 +1,25 @@
+const unified = require('unified')
+const markdown = require('remark-parse')
+const emoji = require('remark-gemoji-to-emoji')
+const remark2rehype = require('remark-rehype')
+const raw = require('rehype-raw')
+const slug = require('rehype-slug')
+const autolinkHeadings = require('rehype-autolink-headings')
+const highlight = require('rehype-highlight')
+const html = require('rehype-stringify')
+const graphql = require('highlightjs-graphql').definer
+const remarkCodeExtra = require('remark-code-extra')
+const codeHeader = require('./plugins/code-header')
+
+module.exports = function createProcessor () {
+  return unified()
+    .use(markdown)
+    .use(remarkCodeExtra, { transform: codeHeader })
+    .use(emoji)
+    .use(remark2rehype, { allowDangerousHTML: true })
+    .use(slug)
+    .use(autolinkHeadings, { behavior: 'wrap' })
+    .use(highlight, { languages: { graphql }, subset: false })
+    .use(raw)
+    .use(html)
+}