🎉 init(cmake): init modern cmake template

docs/mainpage.md

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+# Documentation for Modern Library {#mainpage}
+
+This is the documentation for my simple example library.
+
+It is good documentation because:
+
+1. It exists.
+2. I wrote it.
+3. Everything is documented (pretty easy since there's only one function)
+
+The single provided function is `::accumulate_vector`.