fix(v2): deprecate docs homePageId in favor of frontmatter "slug: /" (#3228)

* homePageId => deprecated

* docs prettier

packages/docusaurus-init/templates/bootstrap/docs/doc1.md

@@ -2,6 +2,7 @@
 id: doc1
 title: Style Guide
 sidebar_label: Style Guide
+slug: /
 ---
 
 You can write content using [GitHub-flavored Markdown syntax](https://github.github.com/gfm/).

packages/docusaurus-init/templates/bootstrap/docusaurus.config.js

@@ -71,8 +71,6 @@ module.exports = {
       '@docusaurus/preset-bootstrap',
       {
         docs: {
-          // It is recommended to set document id as docs home page (`docs/` path).
-          homePageId: 'doc1',
           sidebarPath: require.resolve('./sidebars.js'),
           editUrl:
             'https://github.com/facebook/docusaurus/edit/master/website/',