Make it possible to change sidebar depth per-page

[tbekolay]

Right now it is possible to change the depth of the sidebar by setting sidebar_toc_depth (see https://github.com/nengo/nengo-sphinx-theme/blob/master/nengo_sphinx_theme/theme/sidebar.html#L52). Sometimes, specifically for changelogs it's convenient to set the depth of the sidebar on a page to be different from the global site setting. It should be relatively straightforward to do this on a per-page basis.