docs: add new component docs

docs/src/pages/framed/Modal/Modal.svelte

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+<script>
+  import { Button, Modal } from "carbon-components-svelte";
+
+  let open = false;
+</script>
+
+<Button on:click="{() => (open = true)}">Create database</Button>
+
+<Modal
+  bind:open
+  modalHeading="Create database"
+  primaryButtonText="Confirm"
+  secondaryButtonText="Cancel"
+  on:click:button--secondary="{() => (open = false)}"
+  on:open
+  on:close
+  on:submit
+>
+  <p>Create a new Cloudant database in the US South region.</p>
+</Modal>