docs: add sql service docs section

docs/sql_service.md

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+# Subgraph:SQL Service
+
+The Subgraph:SQL Service, developed by Semiotic Labs in collaboration with The Guild
+and Edge & Node, offers a secure SQL interface for querying a subgraph's entities.
+To deploy this with minimal changes to the existing indexer stack, we wrap a consumer's
+SQL query in a GraphQL query.
+
+## Querying with Subgraph:SQL Service
+
+### Running Queries
+
+Say we have the following SQL query: 
+
+```sql
+SELECT * FROM users WHERE age > 18
+```
+
+The Subgraph:SQL Service allows consumers to create a corresponding GraphQL query
+using the Subgraph:SQL Service `sql` field, with a `query` field containing the SQL
+query:
+
+```graphql
+query {
+  sql(input: {
+    query: "SELECT * FROM users WHERE age > 18",
+    format: JSON
+  }) {
+    ... on SqlJSONOutput {
+      columns
+      rowCount
+      rows
+    }
+  }
+}
+```
+
+We use the `sql` field in the GraphQL query, passing an input object with the SQL
+query, optional parameters, and format. The SQL query selects all columns from the
+`users` table where the `age` column is greater than 18, returning the requested
+data formatted as JSON.
+
+### SQL Parameters and Bind Parameters
+
+#### SQL Query Parameters
+
+You can pass optional SQL query parameters to the SQL query as positional parameters.
+The parameters are converted to the SQL types based on the GraphQL types of the parameters.
+In the GraphQL schema, parameters are passed as an array of `SqlVariable` objects
+within the `parameters` field of the `SqlInput` input object. See the GraphQL schema
+types in `graph/src/schema/sql.graphql`.
+
+#### Bind Parameters
+
+We currently do not support bind parameters, but plan to support this feature in a future
+version of Graph Node.
+
+## Configuration
+
+The Subgraph:SQL Service can be enabled or disabled using the `GRAPH_GRAPHQL_ENABLE_SQL_SERVICE`
+environment variable.
+
+- **Environment Variable:** `GRAPH_GRAPHQL_ENABLE_SQL_SERVICE`
+- **Default State:** Off (Disabled)
+- **Purpose:** Enables queries on the `sql()` field of the root query.
+- **Impact on Schema:** Adds a global `SqlInput` type to the GraphQL schema. The `sql`
+field accepts values of this type.
+
+To enable the Subgraph:SQL Service, set the `GRAPH_GRAPHQL_ENABLE_SQL_SERVICE` environment
+variable to `true` or `1`. This allows clients to execute SQL queries using the
+`sql()` field in GraphQL queries.
+
+```bash
+export GRAPH_GRAPHQL_ENABLE_SQL_SERVICE=true
+```
+
+Alternatively, configure the environment variable in your deployment scripts or
+environment setup as needed.
+
+### SQL Coverage
+
+The Subgraph:SQL Service covers a wide range of SQL functionality, allowing you to execute
+`SELECT` queries against your database. It supports basic querying, parameter binding, and
+result formatting into JSON or CSV. 
+
+#### Whitelisted and Blacklisted SQL Functions
+
+The `POSTGRES_WHITELISTED_FUNCTIONS` constant contains a whitelist of SQL functions that are
+permitted to be used within SQL queries executed by the Subgraph:SQL Service, while `POSTGRES_BLACKLISTED_FUNCTIONS`
+serves as a safety mechanism to restrict the usage of certain PostgreSQL functions within SQL
+queries. These blacklisted functions are deemed inappropriate or potentially harmful to the
+system's integrity or performance. Both constants are defined in `store/postgres/src/sql/constants.rs`.
+
+### SQL Query Validation
+
+Graph Node's SQL query validation ensures that SQL queries adhere to predefined criteria:
+
+- **Function Name Validation**: Validates function names used within SQL queries, distinguishing
+between unknown, whitelisted, and blacklisted functions.
+- **Statement Validation**: Validates SQL statements, ensuring that only `SELECT` queries are
+supported and that multi-statement queries are not allowed.
+- **Table Name Validation**: Validates table names referenced in SQL queries, identifying
+unknown tables and ensuring compatibility with the schema.
+- **Common Table Expression (CTE) Handling**: Handles common table expressions, adding them
+to the set of known tables during validation.
+
+See the test suite in `store/postgres/src/sql/validation.rs` for examples of various scenarios
+and edge cases encountered during SQL query validation, including function whitelisting and
+blacklisting, multi-statement queries, unknown table references, and more.
+
+### Relating GraphQL Schema to Tables
+
+The GraphQL schema provided by the Subgraph:SQL Service reflects the structure of the SQL queries
+it can execute. It does not directly represent tables in a database. Users need to
+construct SQL queries compatible with their database schema.
+
+### Queryable Attributes/Columns
+
+The columns that can be queried depend on the SQL query provided. In the example GraphQL
+query above, the columns returned would be all columns from the `users` table.