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/sales-channels/index.md

@@ -1,3 +1,7 @@
+---
+description: 'Learn what Sales Channels are and how they can be used in Medusa. Sales Channels allow merchants to separate between the different channels products are sold in.'
+---
+
 # Sales Channels
 
 In this document, you’ll learn about Sales Channels and how they can be used in Medusa.

docs/content/advanced/backend/sales-channels/manage-admin.mdx

@@ -1,3 +1,8 @@
+---
+description: 'Learn how to implement sales channels features for admins in Medusa using the REST APIs. This includes how to list sales channels, and create, retrieve, update and delete a sales channel. This also includes how to manage products in a sales channel.'
+addHowToData: true
+---
+
 import Tabs from '@theme/Tabs';
 import TabItem from '@theme/TabItem';