Add ROSCon Spain 2023 ros2_control talk to resources (backport #337)

[mergify]

This PR adds reference to my last year talk on introduction to ros2_control at ROSCon Spain 2023

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This is an automatic backport of pull request #337 done by Mergify.

[mergify]

Cherry-pick of a340cc8 has failed:

On branch mergify/bp/iron/pr-337
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Changes to be committed:
	new file:   doc/resources/presentations/ROSCon_Spain_2023_introduction_to_ros2_control.pdf

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   doc/resources/resources.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally