added skeleton path length example to docs

[hotzenklotz]

Description:

• Added two new examples to the docs:
  • Loading a volume annotation file that was manually downloaded from WK
  • Calculating the path length from a skeleton

Issues:

• None

Todos:

Make sure to delete unnecessary points or to check all before merging:

• Updated Changelog
• Updated Documentation
• Added / Updated Tests
• Considered adding this to the Examples

[normanrz]

Please add tests