docs: change mkdocs theme to 'material'

Signed-off-by: Milosz Wasilewski <milosz.wasilewski@linaro.org>
diff --git a/mkdocs_plugin/requirements.txt b/mkdocs_plugin/requirements.txt
index c8a2de9..53d448e 100644
--- a/mkdocs_plugin/requirements.txt
+++ b/mkdocs_plugin/requirements.txt
@@ -2,3 +2,4 @@
 mdutils
 git+https://github.com/mwasilew/mkdocs-plugin-tags.git
 mkdocs-exclude
+mkdocs-material