chore(docs): Replace homePageId with slug: /

Fix warning ...
The docs plugin option homePageId=intro is deprecated. To make a doc the "home", prefer frontmatter: "slug: /"

Follow-up to #286.

docs/docs/intro.md

@@ -2,6 +2,7 @@
 id: intro
 title: Introduction to ZMK
 sidebar_label: Introduction
+slug: /
 ---
 
 ZMK Firmware is an open source (MIT) keyboard

docs/docusaurus.config.js

@@ -102,7 +102,6 @@ module.exports = {
       {
         docs: {
           // It is recommended to set document id as docs home page (`docs/` path).
-          homePageId: "intro",
           sidebarPath: require.resolve("./sidebars.js"),
           // Please change this to your repo.
           editUrl: "https://github.com/zmkfirmware/zmk/edit/main/docs/",