[Improvement] Reorganize demos (#1052)

docs/markdown/en-US/vant-css.md

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+## Built-in Style
+Vant contains some common styles that can be used directly by the className.
+
+### Text ellipsis
+When the text content length exceeds the maximum container width, the excess text is automatically omitted.
+
+```html
+<div class="van-ellipsis">
+  This is a paragraph of 250px width limit, the back will be omitted.
+</div>
+```
+
+### Hairline
+Add 1px border under the Retina screen for the element, based on a pseudo element.
+
+```html
+<!-- border top -->
+<div class="van-hairline--top"></div>
+
+<!-- border bottom -->
+<div class="van-hairline--bottom"></div>
+
+<!-- border left -->
+<div class="van-hairline--left"></div>
+
+<!-- border right -->
+<div class="van-hairline--right"></div>
+
+<!-- border top & bottom -->
+<div class="van-hairline--top-bottom"></div>
+
+<!-- full border -->
+<div class="van-hairline--surround"></div>
+```
+
+### Animation
+
+```html
+<!-- fade in  -->
+<transition name="van-fade">
+  <div v-show="visible">Fade</div>
+</transition>
+
+<!-- slide bottom -->
+<transition name="van-slide-bottom">
+  <div v-show="visible">Fade</div>
+</transition>
+```