DEVDOCS-6026: [update] add information to section (#617)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6026]


## What changed?
Added introductory information to the Settings API.
* 

## Release notes draft

n/a
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* 

## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->

ping {names}


[DEVDOCS-6026]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6026?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: Roman Liukshyn <[email protected]>

reference/settings.v3.yml

@@ -2,6 +2,30 @@ openapi: '3.0.3'
 info:
   description: |-
     Manage settings and configuration for BigCommerce hosted stores and headless storefronts.
+
+    ## Available settings and configuration
+    | Resource                   | Description                                                                   |
+    |:---------------------------|:------------------------------------------------------------------------------|
+    | Analytics                  | Identifies and updates Web analytics providers.                               |
+    | Catalog                    | Identifies and updates catalog settings.                                      |
+    | Email Statuses             | Identifies and updates transactional email settings.                          |
+    | Favicon Image              | Creates a favicon image.                                                      |
+    | Inventory                  | Identifies and updates inventory settings.                                    |
+    | Inventory Notifications    | Identifies and updates inventory notifications.                               |
+    | Logo                       | Identifies and updates logo settings.                                         |
+    | Logo Image                 | Creates a logo image.                                                         |
+    | Search Filters             | Identifies and updates enabled filters, available filters, and contextual filters.|
+    | Store Locale               | Identifies and updates locale settings.                                        |
+    | Store Profile              | Identifies and updates store profile.                                          |
+    | Storefront Category        | Identifies and updates storefront category settings.                           |
+    | Storefront Product         | Identifies and updates storefront product settings.                            |
+    | Storefront Robots.txt       | Identifies and updates storefront Robots.txt settings.                         |
+    | Storefront Search          | Identifies and updates storefront search settings.                             |
+    | Storefront Security        | Identifies and updates storefront security settings.                           |
+    | Storefront SEO             | Identifies and updates storefront SEO settings.                                |
+    | Storefront Status          | Identifies and updates storefront status.                                      |
+    | Units of Measurement       | Identifies and updates units of measurement settings.                          |
+    
   termsOfService: 'https://www.bigcommerce.com/terms'
   contact:
     name: BigCommerce