doc: jlink plugin description

[vpa1977]

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This PR adds documentation for the jlink plugin: #688

[vpa1977]

Spread test failure:

[sphinx-autobuild] > sphinx-build -c . -b dirhtml . _build -q
matplotlib is not installed, social cards will not be generated
[sphinx-autobuild] Serving on http://0.0.0.0:8080/
[sphinx-autobuild] Waiting to detect changes...
ERROR:    [Errno 98] error while attempting to bind on address ('0.0.0.0', 8080): address already in use
make[1]: *** [Makefile:27: run] Error 1
make[1]: Leaving directory '/rockcraft/docs'
make: *** [Makefile:60: rundocs] Error 2
+ ret=2
+ set -e
+ '[' 2 -ne 124 ']'
+ exit 2