doc: add nitpicky mode ignores

The nitpicky mode warns about any broken reference. It is useful to keep
docs in good shape, however, there are some references that will never
be resolvable. Provide an initial list to decrease the number of
warnings that are not relevant/can't be fixed.

Note that the Sphinx nitpicky mode can be enabled by either setting
`nitpicky = True` in `doc/conf.py` or by setting the `-n` flag in
`SPHINXOPTS`.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>