Fix links in docs

[christophfroehlich]

There are two broken links

I haven't found any replacement for the GDB CLI handout, @firesurfer do you have an alternative resource by chance?

[saikishor]

Suggested change

	* `Using GDB to debug a plugin <https://stackoverflow.com/questions/10919832/how-to-use-gdb-to-debug-a-plugin>`_
	* `Using GDB to debug a plugin <https://stackoverflow.com/questions/10919832/how-to-use-gdb-to-debug-a-plugin>`_
	* `GDB CLI Tutorial <https://www.cs.umd.edu/~nelson/classes/resources/cdebugging/gdb/gdb-reference-card.pdf>`_

[saikishor]

Suggested change

	* `Using GDB to debug a plugin <https://stackoverflow.com/questions/10919832/how-to-use-gdb-to-debug-a-plugin>`_
	* `Using GDB to debug a plugin <https://stackoverflow.com/questions/10919832/how-to-use-gdb-to-debug-a-plugin>`_
	* `GDB CLI Tutorial <https://users.ece.utexas.edu/~adnan/gdb-refcard.pdf>`_

Maybe, we can use one of these two.

Do you think it makes sense to host these files locally in github and cross-reference them?

[christophfroehlich]

I don't know what was the content of the old link, the new ones are no "tutorial"

[christophfroehlich]

about hosting them, idk about the copyright. I don't want to do that without the permission of the authors.

[saikishor]

I remember it being like a cheatsheet. May be I'm wrong. Let's see what @firesurfer @fmauch says