Update docs structure

Description

• Currently, the docs have many unorganized design docs in the main navigation: https://autowarefoundation.gitlab.io/autoware.auto/AutowareAuto/. This sorts them into their correct top-level packages.
• Removed the package-design suffix from the link names following the example of other docs (e.g. localization)
• Fixed the object collision estimator heading hierarchy

Notes for Reviewer

• Tested with docs/.doxygen/build.py
• We could have a separate subdir for planning instead of lumping it all into motion.
• Should the package with the node in it be called *_node or *_nodes if there is just one?
• perception docs have subpages links in the form md_src_perception_filters_voxel_grid_design_voxel-grid-filter-design, which one is better?

Checklist

• Documentation was updated