docs/devel/testing: Dissolve the ci-definitions.rst.inc file

docs/devel/testing: Dissolve the ci-definitions.rst.inc file

This file was meant for defining the vocabulary for our testing
efforts, but it did not age well: First, the definitions are not
only about the CI part, but also about testing in general, so most
of the information should rather reside in main.rst instead.
Second, some vocabulary has never been really adopted by the QEMU
project, for example we never really use the word "system testing"
since "system" rather means the system emulator binaries in the
QEMU project (and we also don't do any testing with other components
like libvirt and virt-managers here). It also defines that the qtests
are the "functional" tests in QEMU, which is not really up to date
anymore after the "tests/functional" framework has been introduced
a couple of months ago (FWIW, the qtests could rather be seen as a
mix between unit testing and functional testing).

To solve this problem, move the useful parts of this file into
main.rst and directly into ci.rst, and drop the ones (like "system
testing") that we don't really need anymore.

Message-ID: <20250314085959.1585568-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>