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/starters/gatsby-medusa-starter.mdx

@@ -1,3 +1,7 @@
+---
+description: 'Learn how to install the Gatsby starter storefront to use with the Medusa Server. The Gatsby storefront includes features related to customer profiles, order returns and exchanges, and more.'
+---
+
 import Feedback from '@site/src/components/Feedback';
 
 # Gatsby Storefront Quickstart

docs/content/starters/nextjs-medusa-starter.mdx

@@ -1,3 +1,7 @@
+---
+description: 'Learn how to install the Next.js starter storefront to use with Medusa. The Next.js storefront includes ready-integrations with plugins like Stripe and Algolia. It offers features like customer accounts, cart and checkout flows, and more.'
+---
+
 import Feedback from '@site/src/components/Feedback';
 
 # Next.js Storefront Quickstart