Update requirements.txt

Add markdown and pygments and sort

docs/requirements.txt

@@ -1,8 +1,10 @@
-mkdocs==1.1.2
+git+git://github.com/g-provost/lightgallery-markdown#egg=lightgallery
-mkdocs-material==7.1.7
+markdown==3.3.4
-pymdown-extensions==8.2
-mkdocs-macros-plugin==0.5.5
 mkdocs-awesome-pages-plugin==2.5.0
-mkdocs-redirects==1.0.3
 mkdocs-git-revision-date-localized-plugin==0.9.2
-git+git://github.com/g-provost/lightgallery-markdown#egg=lightgallery
+mkdocs-macros-plugin==0.5.5
+mkdocs-material==7.1.7
+mkdocs-redirects==1.0.3
+mkdocs==1.1.2
+pygments==2.9.0
+pymdown-extensions==8.2