Minor fixes in documentation

[YBachmann]

• Outdated Table of Contents:
  • The ToC in all CONTRIBUTING.md files was outdated with some dead links and some missing entries → Updated table of contents
• Incorrect Imports in Documentation
  • Some imports in the documentation "Creating Node Classes" were incorrect.
  • Despite not being used: import rospy seems to be a leftover from the ROS1 package → replaced with import rclpy
  • from ros_bt_py_msgs.msg import Node as NodeMsg is not the correct import → replaced with from ros_bt_py_interfaces.msg import Node as NodeMsg (on another documentation site ("Testing Node Classes") the correct import is already used

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 32.93%. Comparing base (87e731a) to head (5b21c63).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #62 +/- ## ======================================= Coverage 32.93% 32.93% ======================================= Files 37 37 Lines 5404 5404 Branches 1249 1249 ======================================= Hits 1780 1780 Misses 3556 3556 Partials 68 68 ```

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