refactor(quadratic.md): 重新组织二次域页面内容 (OI-wiki#6100)

* refactor(quadratic.md): organize and rewrite

* fix: apply suggestions

Author: c-forrest

docs/math/number-theory/images/eisenstein-integer.svg

@@ -0,0 +1,24 @@
+\documentclass[12pt]{standalone}
+\usepackage{amsmath,amssymb}
+\usepackage{newtxtext,newtxmath}
+\usepackage{tikz}
+\begin{document}
+\begin{tikzpicture}[scale=1.5]
+\clip (0.8,1.8) rectangle (7.6,6.4);
+\def\nx{10}
+\def\ny{4} 
+\foreach \j in {0,...,\ny} {
+    \foreach \i in {0,...,\nx} {
+        \draw[dashed](0:\i)++(90:{(1+2*\j)*sin(60)}) -- ++(1,0);
+        \draw[dashed](0:\i)++(60:\j)++(120:\j) -- ++(60:2) 
+                        -- ++(-1,0) -- ++(-60:1) -- ++(-60:1);
+    }
+}
+\draw[->,thick] (0.8,3.464) -- (7.6,3.464) node[below left]{Re};
+\draw[->,thick] (4,1.8) -- (4,6.4) node[below left]{Im};
+\draw (4,3.464) node[below left]{$0$};
+\draw (5,3.464) node[below]{$1$};
+\draw[fill=black] (3.5,4.33) circle (1.25pt) node[below]{$\omega$};
+\draw[fill=black] (6,5.196) circle (1.25pt) node[below left]{$3+2\omega$};
+\end{tikzpicture}
+\end{document}

docs/math/number-theory/images/eisenstein-integer.tex

@@ -0,0 +1,15 @@
+\documentclass[12pt]{standalone}
+\usepackage{amsmath,amssymb}
+\usepackage{newtxtext,newtxmath}
+\usepackage{tikz}
+\begin{document}
+\begin{tikzpicture}[scale=1.5]
+\draw[dashed] (-3.8,-2.4) grid (3.8,3.8);
+\draw[->,thick] (-3.8,0) -- (3.8,0) node[below left] {Re};
+\draw[->,thick] (0,-2.4) -- (0,3.8) node[below left] {Im};
+\draw (0,0) node[below left] {$0$};
+\draw (0,1) node[below left] {$\mathrm{i}$};
+\draw (1,0) node[below left] {$1$};
+\draw[fill=black] (3,2) circle (2pt) node[above left] {$3+2\mathrm{i}$}; 
+\end{tikzpicture}
+\end{document}