sysctl changes for v6.9-rc1

I'm sending you the sysctl pull request after following Luis' suggestion to
become a maintainer. If you see that something is missing, get back to me with
how to improve and I'll include your feedback in the following PRs.

Here is a summary of the changes included in this PR:
* New shared repo for sysctl maintenance
* check-sysctl-docs adjustment for API changes by Thomas Weißschuh

This is a non-functional PR. Additional testing is required for the rest of the
pending changes. Future kernel pull requests will include the removal of the
empty elements (sentinels) from sysctl arrays in the kernel/, net/, mm/ and
security/ dirs. After that, the superfluous check for procname == NULL will be
removed. And the push to avoid bloating the kernel as these arrays move out of
kernel/sysctl.c will be completed.

Even though Thomas' changes went into sysctl-next after v6.8-rc5 (3 weeks in
linux-next), I include them as they contained no functional changes and
therefore have little chance of resulting in an error/regression. Finally the
new shared repo is now picked up by linux-next and is the source for upcoming
sysctl changes.
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