add command pip install requirements.txt in build step

doc/requirements.txt

@@ -1 +1,11 @@
-mkdocs>=1.1.2
+mkdocs
+mkdocs-bootswatch
+mkdocs-material
+mkdocs-material-extensions
+fontawesome-markdown
+pymdown-extensions
+mdx_truly_sane_lists
+mkdocs-git-revision-date-localized-plugin
+mkdocs-git-revision-date-plugin
+mkdocs-minify-plugin
+mkdocs-add-number-plugin

requirements.txt

@@ -1,11 +0,0 @@
-mkdocs
-mkdocs-bootswatch
-mkdocs-material
-mkdocs-material-extensions
-fontawesome-markdown
-pymdown-extensions
-mdx_truly_sane_lists
-mkdocs-git-revision-date-localized-plugin
-mkdocs-git-revision-date-plugin
-mkdocs-minify-plugin
-mkdocs-add-number-plugin