Improve performances of sidebar generation and loading

[tristanlatr]

With the side bar generation, especially on project that contain lots of classes per modules and with a value higher than 1 for option --sidebar-expand-depth, files can rapidly take a few megas and be really long to load, even locally. To ensure the best user experience no matter the value of --sidebar-expand-depth, we should generate the sidebars for each documentable in a separate file (i.e. .sidebar.html) and load it with JS when the rest of the content has been loaded.

Links:

• Sidebar #370 (comment)
• Sidebar #370 (comment)