Add "Features" subsection to website to add feature descriptions

[SteveMacenski]

• dynamic following
• point to point navigation Add dynamic point following documentation as a feature set / core feature descriptions #56
• perception processing Add descriptions of sensor processing / perception options in core navigation descriptions #55 (2D, 3D camera, dynamic, radar, sonar, 3D lidar, etc)
• planners
• controllers
• custom BT nodes + BT navigator logic
• custom BT XML autonomy, or use your own state machine system instead if you like.
• autonomous navigation
• lidar localization
• map serving
• recoveries
• rviz plugins
• system tests
• real-time / production support (lifecycle, bond, etc)
• Testing (nav test pkg, rviz plugin, bringup), linting, static analysis
• waypoint following + plugin for applications
• ackermann, differential, omni, legged; round or arbitrary
• multiple planning and controllers Create Planner/Controller selector BT nodes  navigation2#2086 either in single task or between tasks (selector nodes to change, BT context change hardcoded, or new BT between requests to vary)
• keepout and speed restricted
• gps
• vlsam
• 3d SLAM
• variable precision navigation requests (goal checkers)
• navigate through N poses as well as navigate to a pose (for via points not stopping. For stopping, use WP follower)
• Removable recovery/controller/planner servers for other langauges, plugin interfaces, env reps, distribute across multiple CPUs, distribute to the cloud, multiple instances of each, flexibility
• Plugins for core algorithms to separate robotics algorithm developers from robotics software developers.
• Use of BT nodes and mapped names to separate robotics software developers from robotics application developers to build custom applications without worrying about specific details
• Supporting N algorithm plugins
• Massive library of Bt nodes to use to compose systems
• Semantic representations + router server to use to navigate in large / outdoor spaces
• Dynamic tracking of multiple modalities in controllers/planners/perception pipelines
• localization framework multi-modal building block, better than AMCL
• map editing tools / semantic labeling visualizers
• height modelling + swappable environmental models in all servers
• Outdoor nav with route server + height models
• ... etc

[SteveMacenski]

Videos from ROS world

• 2 Siemen's videos keepout and BT groot
• 2 keepout videos https://www.youtube.com/watch?v=IZnkLtdv3BU https://www.youtube.com/watch?v=JumHmprSrcQ
• hybrid A* from Mac https://www.youtube.com/watch?v=vaxNA98j0LQ
• dynamic object (sim and real) https://www.youtube.com/watch?v=wLBQPPPuT-M
• speed retricted zones https://www.youtube.com/watch?v=xgK1N5ye5Rg&feature=youtu.be

[SteveMacenski]

Description pages for each package with videos / images