doc: Add a explanation of Git's data model #25361

Job	Run time
config	7s
fuzz smoke test	2m 13s
win+VS build	0s
win build	1s
win+Meson build	2s
static-analysis	1s
documentation	1s
sparse	1s
rust-analysis	1s
osx-clang (macos-14)	2s
osx-reftable (macos-14)	2s
osx-gcc (macos-14)	3s
linux-reftable-leaks (ubuntu:rolling)	3s
osx-meson (macos-14)	3s
almalinux-8 (almalinux:8)	1s
debian-11 (debian:11)	1s
linux32 (i386/ubuntu:focal)	1s
linux-sha256 (ubuntu:rolling)	1s
linux-reftable (ubuntu:rolling)	1s
linux-leaks (ubuntu:rolling)	3s
linux-meson (ubuntu:rolling)	3s
linux-asan-ubsan (ubuntu:rolling)	3s
fedora-breaking-changes-meson (fedora:latest)	1s
linux-breaking-changes (ubuntu:rolling)	1s
linux-musl-meson (alpine:latest)	1s
linux-TEST-vars (ubuntu:20.04)	1s
win+VS test	0s
win+Meson test	0s
win test	0s
	2m 58s

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