docs: improved SEO of documentation (#3117)

* docs: added description to documentation pages

* docs: added more descriptions

* docs: finished improving meta description

* docs: added searchbox structured data

* docs: added breadcrumbs structured data

* docs: added how to structured data

* docs: improved 404 page

* docs: added how-to frontmatter option

docs/content/advanced/backend/upgrade-guides/index.mdx

@@ -1,5 +1,6 @@
 ---
 hide_table_of_contents: true
+description: 'Upgrade guides on how to update the Medusa server along with other Medusa components to the latest version.'
 ---
 
 import DocCardList from '@theme/DocCardList';