docs: add placeholder welcome guide

Author: graynorton

2nd-gen/packages/swc/.storybook/guides/welcome.mdx

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+import { Meta } from '@storybook/addon-docs/blocks';
+
+<Meta title="Welcome to 2nd-gen SWC" />
+
+# Welcome to 2nd-gen SWC
+
+This is the Storybook for the 2nd generation (2nd-gen) of Spectrum Web Components (SWC).
+
+## About Spectrum Web Components
+
+Spectrum Web Components (SWC) is a library of [web components](https://developer.mozilla.org/docs/Web/Web_Components) that implements [Adobe's Spectrum design system](https://spectrum.adobe.com/).
+
+While SWC is used primarily by Adobe product teams, it is open-sourced and available for general use.
+
+SWC is developed by a core team in Adobe Design Engineering, but we welcome contributions from inside and outside Adobe.
+
+## About 2nd-gen
+
+SWC is currently in transition from its first generation (1st-gen) to its second generation (2nd-gen).
+
+2nd-gen has two main goals:
+
+- To provide a clean foundation for the future of SWC
+
+- To deliver a full-fidelity Spectrum 2 implementation
+
+2nd-gen SWC is currently in active development, with the first pre-releases expected in early 2026.
+
+For more information on 2nd-gen plans and status, see the [CONTRIBUTOR-DOCS](https://github.com/adobe/spectrum-web-components/tree/barebones/CONTRIBUTOR-DOCS/README.md) in the [SWC repository](https://github.com/adobe/spectrum-web-components).
+
+## About these Guides
+
+Eventually, this **Guides** section will contain non-component-specific docs for 2nd-gen SWC users, replacing 1st-gen SWC's standalone [doc site](https://opensource.adobe.com/spectrum-web-components/).