docs: create mdbook

doc/.gitignore

@@ -0,0 +1 @@
+book

doc/book.toml

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+[book]
+authors = []
+language = "en"
+multilingual = false
+src = "src"
+title = "Revolt Backend"

doc/src/SUMMARY.md

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+# Summary
+
+- [Introduction](./hello.md)
+- [Project Structure]()
+- [Testing]()
+  - [Writing a new database test]()
+  - [Writing a new API test]()

doc/src/hello.md

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+# Revolt Backend
+
+Welcome to the developer documentation for the Revolt backend.
+
+This is very much incomplete and needs more work!