add page_description and page_keywords md meta to mkdocs html template

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5516 from SvenDowideit/docs-meta-rendering

add page_description and page_keywords md meta to mkdocs html template