Fix API docs: add missing methods and Animation method listing

- Add Dataset.traj.concave_hull (was in code but missing from docs)
- Add Dataset.traj.insert_nan_where and drop_where (Traj2d methods)
- Explicitly list all Animation public methods (build, show, save,
  color_by, show_trajectories, set_fps, set_markersize, set_title,
  overlay_variable) replacing the stale auto-detected stub

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gauteh

docs/source/api.rst

@@ -119,13 +119,16 @@ Methods
     Dataset.traj.skill
     Dataset.traj.velocity_components
     Dataset.traj.velocity_spectrum
+    Dataset.traj.concave_hull
     Dataset.traj.convex_hull
     Dataset.traj.convex_hull_contains_point
     Dataset.traj.get_area_convex_hull
     Dataset.traj.gridtime
     Dataset.traj.sel
     Dataset.traj.seltime
     Dataset.traj.iseltime
+    Dataset.traj.insert_nan_where
+    Dataset.traj.drop_where
     Dataset.traj.append
     Dataset.traj.crop
     Dataset.traj.contained_in
@@ -150,6 +153,25 @@ Plotting
    plot.Plot
    animation.Animation
 
+.. currentmodule:: trajan.animation
+
+Animation methods
+~~~~~~~~~~~~~~~~~
+
+.. autosummary::
+   :toctree: generated/
+   :template: autosummary/accessor_method.rst
+
+   Animation.color_by
+   Animation.show_trajectories
+   Animation.set_fps
+   Animation.set_markersize
+   Animation.set_title
+   Animation.overlay_variable
+   Animation.build
+   Animation.show
+   Animation.save
+