The key is given on the keys page and you can download it using gpg --recv 875BE862
$ gpg --recv-key 875BE862
gpg: key 131910E01605D9AA: "Mukunda Modell (WMF) <mmodell@wikimedia.org>" not changed
gpg: Total number processed: 1
gpg: unchanged: 1
$ gpg --list-key 875BE862
pub rsa4096 2016-01-08 [SC] [expires: 2020-07-31]
C83A8E4D3C8FEB7C8A3A1998131910E01605D9AA
uid [ undef ] Mukunda Modell (WMF) <mmodell@wikimedia.org>
uid [ unknown] Mukunda Modell <twentyafterfour@gmail.com>
uid [ undef ] [jpeg image of size 2928]
sub rsa4096 2018-12-25 [A] [expires: 2020-12-24]
sub rsa4096 2018-12-25 [E] [expires: 2020-12-24]
sub rsa4096 2018-12-26 [S] [expires: 2019-12-26]
$ gpg --verify mediawiki-1.32.0-rc.0.tar.gz.sig
gpg: assuming signed data in 'mediawiki-1.32.0-rc.0.tar.gz'
gpg: Signature made Fri 09 Nov 2018 03:52:36 PM EST
gpg: using RSA key 131910E01605D9AA
gpg: checking the trustdb
gpg: marginals needed: 3 completes needed: 1 trust model: classic
gpg: depth: 0 valid: 4 signed: 25 trust: 0-, 0q, 0n, 0m, 0f, 4u
gpg: depth: 1 valid: 23 signed: 33 trust: 22-, 0q, 0n, 1m, 0f, 0u
gpg: next trustdb check due at 2019-01-29
gpg: Good signature from "Mukunda Modell (WMF) <mmodell@wikimedia.org>" [full]
gpg: aka "Mukunda Modell <twentyafterfour@gmail.com>" [unknown]
gpg: aka "[jpeg image of size 2928]" [full]
I trust Modell's key because it is signed by Grossmeier's and Gossmeier and I have signed each other's keys in person.