Adding CNAME file (#51)

## Description
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## Motivation and Context
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## Screenshots (if appropriate):
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## Types of changes
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- [ ] Edits to existing documentation
- [ ] Changing documentation structure (relocating existing files,
ensure redirects exist)
- [ ] Stylistic changes (provide screenshots above)

docs/public/CNAME

@@ -0,0 +1 @@
+docs.astria.org