doc: Use standlone requirements.txt

Author: SilverRainZ

doc/requirements.txt

@@ -0,0 +1,11 @@
+# deps of sphinxnotes-snippet
+langid
+jieba
+python-pinyin
+pyxdg
+stopwordsiso
+wcwidth
+wordsegment
+
+# deps of documentation
+sphinxcontrib.asciinema