doc: releases: known issues for v3.1.0

Every release notes page includes a section for known issues.

In previous releases, we have linked to a GitHub query which displays
open issues. The problem with this approach is that the information
immediately goes stale, making this section's content less than useful.

We can do better now that we have a bug snapshot workflow and a script
capable of dumping snapshot contents in the same almost-rst-format
used by list_issues.py.

Use it as follows to archive the known open bugs at time of release:

$ wget https://builds.zephyrproject.io/zephyr/bug-snapshot/zephyr-bugs-2022-06-03.pickle.xz


$ unxz zephyr-bugs-2022-06-03.pickle.xz
$ ./scripts/dump_bugs_pickle.py zephyr-bugs-2022-06-03.pickle

Then copy paste the output into the release notes and handle .rst
escapes in the same way has previously been done for closed issues
since the last release.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>