doc(alphatex): add vendor alphaTex docs to ./docs and ./data to include official introduction in tutorials

docs/vendor/alphaTabWebsite/README.md

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+# alphaTabWebsite (vendor copy)
+
+This directory contains a vendor copy of the alphaTab project's documentation (the "AlphaTeX" docs) pulled from the node_modules/alphaTabWebsite package.
+
+Source: https://github.com/CoderLine/alphaTabWebsite
+
+Files were copied with the script: 
+
+    node scripts/vendor-alphatex.cjs
+
+**License & attribution:** Upstream project license has been copied into this folder as `LICENSE`. Please refer to it for license terms. Original authors and contributors are preserved in the upstream project's repository.

docs/vendor/alphaTabWebsite/UPSTREAM_README.md

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+# alphaTab Website and Documentation
+
+![alphaTab](https://raw.githubusercontent.com/CoderLine/alphaTab/develop/img/banner.png)
+
+This repository contains the website and documentation of alphaTab. Head over to the main product repository to learn more.
+
+Product Repository: <https://github.com/CoderLine/alphaTab>
+Website: <https://www.alphatab.net/>
+
+We are using <img alt="Docusaurus" src="https://docusaurus.io/img/docusaurus.svg" width="16" /> [Docusaurus](https://docusaurus.io/) for the docs.