Automate sidebar navigation from `.yaml` files as plugin

[aileen]

Problem description

The currently implemented solution to generate the sidebar navigation from yaml files reads all yaml files from within a /src//data/sidebars directory, which is weird because those files are part of the content and not the code and currently stored away from the content files.

Those sidebar files should actually live right where they belong: in the related content folder.

Proposal

Refactor the sidebars components to be more automatic and generic and read them from the content directories.

Create this new implementation as plugin, so it can be reused.

Todos

• move all sidebar files out of /src into their associated content dir
• create a plugin that finds those files and can be simply used as Sidebar component