docs: Add a link to OpenAPI docs

Author: igorlukanin

docs/pages/product/apis-integrations/ai-api.mdx

@@ -11,6 +11,8 @@ Specifically, you can send the AI API a message (or conversation of messages) an
   your Cube representative to have it enabled for your account.
 </WarningBox>
 
+See [AI API reference][ref-ref-ai-api] for the list of supported API endpoinsts.
+
 ## Configuration
 
 While the AI API is in preview, your Cube account team will enable and configure it for you.
@@ -217,4 +219,7 @@ deployment. See below for required variables by provider (required unless noted)
 #### OpenAI
 
 - `OPENAI_MODEL` - An OpenAI chat model ID, for example `gpt-4o`
-- `OPENAI_API_KEY` - An OpenAI API key (we recommend creating a service account for the AI API)
+- `OPENAI_API_KEY` - An OpenAI API key (we recommend creating a service account for the AI API)
+
+
+[ref-ref-ai-api]: /reference/ai-api

docs/pages/product/apis-integrations/rest-api.mdx

@@ -16,6 +16,16 @@ data applications, including but not limited to the following ones:
 Often, the REST API is used to enable [embedded analytics][cube-ea] and
 [real-time analytics][cube-rta] use cases.
 
+See [REST API reference][ref-ref-rest-api] for the list of supported API endpoinsts.
+Also, check [query format][ref-rest-query-format] for details about query syntax.
+
+<InfoBox>
+
+You can find a mostly complete OpenAPI documentation for the REST API in the
+linked [`openspec.yml` file][gh-cube-openspec] that you can use with tools like Swagger.
+
+</InfoBox>
+
 <InfoBox>
 
 If you've chosen [GraphQL][graphql] as a query language for your front-end
@@ -24,10 +34,9 @@ provides.
 
 </InfoBox>
 
-Under the hood, REST API also provides endpoints for [GraphQL
-API][ref-graphql-api] and [Orchestration API][ref-orchestration-api]. However,
-they target specific use cases and are not usually considered part of the REST
-API.
+REST API also provides endpoints for [GraphQL API][ref-graphql-api] and
+[Orchestration API][ref-orchestration-api]. However, they target specific use
+cases and are not considered part of the REST API.
 
 ## Example request
 
@@ -84,10 +93,6 @@ curl \
   http://localhost:4000/cubejs-api/v1/meta | jq
 ```
 
-Check [query format][ref-rest-query-format] for details about supported queries.
-Also, check [REST API reference][ref-ref-rest-api] for the list of supported API
-endpoints.
-
 ## Configuration
 
 REST API is enabled by default and secured using [API scopes][self-api-scopes]
@@ -291,3 +296,4 @@ example, the following query will retrieve rows 101-200 from the `Orders` cube:
 [self-cors]: #configuration-cors
 [ref-ref-rest-api]: /reference/rest-api
 [link-jq-utility]: https://jqlang.github.io/jq/
+[gh-cube-openspec]: https://github.com/cube-js/cube/blob/master/packages/cubejs-api-gateway/openspec.yml

docs/pages/product/apis-integrations/sql-api.mdx

@@ -20,6 +20,10 @@ but not limited to the following ones:
 Often, the SQL API is used to enable internal or self-serve [business
 intelligence][cube-bi-use-case] use cases.
 
+See [SQL API reference][ref-ref-sql-api] for the list of supported SQL commands,
+functions, and operators. Also, check [query format][ref-sql-query-format] for
+details about supported queries.
+
 <InfoBox>
 
 Please use [this GitHub issue](https://github.com/cube-js/cube/issues/3906) to
@@ -78,10 +82,6 @@ WHERE table_schema = 'public';
 \d
 ```
 
-Check [query format][ref-sql-query-format] for details about supported queries.
-Also, check [SQL API reference][ref-ref-sql-api] for the list of supported SQL
-commands, functions, and operators.
-
 ## Fundamentals
 
 In the SQL API, each cube or view from the [data model][ref-data-model-concepts]