Update Sphinx presentation plugin
Currently the OSGeoLive docs generate a presentation based on revealJS using the plugin sphinxjp.themes.revealjs - https://github.com/tell-k/sphinxjp.themes.revealjs
This plugin has not been updated for a couple of years and only works on sphinx==1.6.5.
A newer plugin which should also generate a presentation can be found at https://github.com/attakei/sphinx-revealjs and works with Sphinx > 3.0.0. Updating OSGeoLive-docs to use this plugin will allow using more recent versions of Sphinx.