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 https://github.com/gazebo-web/gazebosim-web-frontend/issues/20). Lastly, changes to the docs repo requires manual deployment of https://github.com/gazebo-web/gazebosim-web-backend/ (see https://github.com/gazebo-web/gazebosim-web-backend/issues/8). To mitigate all of these issues and address some feature requests (https://github.com/gazebo-web/gazebosim-web-frontend/issues/24, https://github.com/gazebo-web/gazebosim-web-frontend/issues/21, https://github.com/gazebo-web/gazebosim-web-frontend/issues/9) and others, we should migrate to using a static site generator.