docs: project documentation layout refactored

docs/appendix/release_notes.md

@@ -1 +0,0 @@
-../../CHANGELOG.md

docs/index.md

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+---
+hide:
+  - navigation
+  - toc
+---
+
 # Welcome to **mp-units**!
 
 **mp-units** is a compile-time enabled feature-rich Modern C++ modular/header-only library that

docs/library_reference/core_library.md

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-# Core Library
-
-## Concepts
-
-## Expression templates
-
-## Dimensions

docs/release_notes.md

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+../CHANGELOG.md