Use a static site generator for gazebosim.org

[azeey]

The current architecture of the gazebosim.org website makes it difficult to preview changes locally (see #85). It also has shortcomings when it comes to SEO (see gazebo-web/gazebosim-web-frontend#20). Lastly, changes to the docs repo requires manual deployment of https://github.com/gazebo-web/gazebosim-web-backend/ (see gazebo-web/gazebosim-web-backend#8). To mitigate all of these issues and address some feature requests (gazebo-web/gazebosim-web-frontend#24, gazebo-web/gazebosim-web-frontend#21, gazebo-web/gazebosim-web-frontend#9) and others, we should migrate to using a static site generator.