docs: Switch out pkg reference link

[AnnsAnns]

Contribution description

Currently the pkg documentation @ https://doc.riot-os.org/group__pkg.html has a link that is supposed to point to https://doc.riot-os.org/creating-modules.html#modules-outside-of-riotbase but on the Doxygen Output it simply creates a dead (and confusing) link to https://doc.riot-os.org/src/creating-modules.html#modules-outside-of-riotbase.

Simply replacing it with the full link should cover both people using the doxygen documentation and people checking out the doc.txt itself.

Testing procedure

Issues/PRs references

[crasbe]

The section modules-outside-of-riotbase actually has a label, so you can access it with the @ref command.

[external modules](@ref modules-outside-of-riotbase) creates a link to html/creating-modules.html#modules-outside-of-riotbase. Your solution would not work on a local machine. The full link for me locally is file://wsl.localhost/Ubuntu-22.04/home/cbuec/riot-xipfs/RIOT/doc/doxygen/html/creating-modules.html#modules-outside-of-riotbase.

With the @ref command, it works either way.

[AnnsAnns]

I will adjust this, thank you for the feedback