add docs about new database modules

src/content/docs/modules/db/mysql.mdx

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+---
+title: MySQL
+sidebar:
+  order: 1
+---
+import SchemaInfo from '../../../../components/SchemaInfo.astro';
+
+The `db.mysql` module can open a connection to MySQL databases
+
+<SchemaInfo type="module" id="db.mysql"></SchemaInfo>
+
+This module implements the `DatabaseModule` interface.
+
+
+### Example
+
+Opens a connection to a MySQL database
+
+```yaml
+- id: myDatabase
+  type: db.mysql
+  params:
+    dsn: "user:password@tcp(localhost:3306)/mydatabase"
+```

src/content/docs/modules/db/postgres.mdx

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+---
+title: PostgreSQL
+sidebar:
+  order: 1
+---
+import SchemaInfo from '../../../../components/SchemaInfo.astro';
+
+The `db.postgres` module can open a connection to PostgreSQL databases
+
+<SchemaInfo type="module" id="db.postgres"></SchemaInfo>
+
+This module implements the `DatabaseModule` interface.
+
+
+### Example
+
+Opens a connection to a PostgreSQL database
+
+```yaml
+- id: myDatabase
+  type: db.postgres
+  params:
+    url: "postgresql://user:password@localhost:5432/mydatabase"
+```