refactor(docs): order requirements file alphabetically

requirements.txt

+certifi~=2019.9
+markdown_del_ins~=1.0
 Markdown~=3.1
-mkdocs~=1.0
 mkdocs-material~=4.0
-plantuml~=0.2
+mkdocs~=1.0
 plantuml-markdown~=3.1
+plantuml~=0.2
 pymdown-extensions~=6.2
-certifi~=2019.9
-markdown_del_ins~=1.0