Update 3D character examples link

[ostin-r]

Objective

Update broken links in RigidBody documentation, at the bottom of the Movement section:
https://docs.rs/avian3d/latest/avian3d/dynamics/rigid_body/enum.RigidBody.html#movement

Broken links are displayed as basic_kinematic_character and basic_dynamic_character.

Solution

No breaking changes.

Replaced the broken links with a single link to the examples directory in the avian3d crate, using a github permalink. I am confused why the original links broke as they appear to be permalinks as well. Maybe something damaged the git history where these links originally lived - a rebase, or some other damage to the history.

Testing

Changes were tested (and can be tested by reviewers) by generating the docs with cargo:
cargo doc -p avian3d
and jumping to <project_dir>/target/doc/avian3d/dynamics/rigid_body/enum.RigidBody.html#movement

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