added doc indexes

docs/events/index.md

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+# Events
+
+A reference to Textual [events](../guide/events.md).

docs/guide/index.md

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+# Textual Guide
+
+Welcome to the Textual Guide! An in-depth reference on how to build app with Textual.
+
+## Example code
+
+Most of the code in this guide is fully working—you could cut and paste it if you wanted to.
+
+Although it is probably easier to check out the [Textual repository](https://github.com/Textualize/textual) and navigate to the `docs/examples/guide` directory and run the examples from there.

docs/how-to/index.md

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+# How to ...
+
+For those who want more focused information on Textual features.

docs/reference/index.md

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+# Reference
+
+A reference to the Textual public APIs.

docs/styles/index.md

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+# Styles
+
+A reference to Widget [styles](../guide/styles.md).  

docs/widgets/index.md

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+# Widgets
+
+A reference to the builtin [widgets](../guide/widgets.md).