Switch opennav_docking_core to modern CMake idioms.

[clalancette]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Follow-up to #4357
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

Update opennav_docking_core to modern CMake idioms:

1. Add in an INTERFACE target that contains the header files
2. Export a CMake target so downstream packages can use it.
3. Reduce the exported dependencies.
4. Push the include directory down one level, which is best practice since Humble.

Description of documentation updates required from your changes

None needed.

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Future work that may be required in bullet points

This is part of a larger series to convert Navigation2 to modern CMake idioms. There are 6 PRs required after this one.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists