doc: Add page describing OS distros used by the CI

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: I9efbb1928e1935ec55edbdb23a6ad99772b1e6ae
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9522
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>

doc/Doxyfile

@@ -814,6 +814,7 @@ INPUT                  += \
                          concurrency.md \
                          containers.md \
                          ../deprecation.md \
+                         distributions.md \
                          event.md \
                          ftl.md \
                          gdb_macros.md \

doc/ci_tools.md

@@ -4,3 +4,4 @@ Section describing tools used by CI to verify integrity of the submitted
 patches ([status](https://ci.spdk.io)).
 
 - @subpage shfmt
+- @subpage distributions

doc/distributions.md

@@ -0,0 +1,64 @@
+# distributions {#distributions}
+
+## In this document {#distros_toc}
+
+* @ref distros_overview
+* @ref linux_list
+* @ref freebsd_list
+
+## Overview {#distros_overview}
+
+CI pool uses different flavors of `Linux` and `FreeBSD` distributions which are
+used as a base for all the tests run against submitted patches. Below is the
+listing which covers all currently supported versions and the related CI
+jobs (see [status](https://ci.spdk.io) as a reference).
+
+## Linux distributions {#linux_list}
+
+* Fedora: Trying to follow new release as per the release cycle whenever
+          possible. Currently at `Fedora33`.
+
+```list
+- autobuild-vg-autotest
+- clang-vg-autotest
+- iscsi*-vg-autotest
+- nvme-vg-autotest
+- nvmf*-vg-autotest
+- scanbuild-vg-autotest
+- unittest-vg-autotest
+- vhost-initiator-vg-autotest
+```
+
+Jobs listed below are run on bare-metal systems where version of
+Fedora may vary. In the future these will be aligned with the
+`vg` jobs.
+
+```list
+- BlobFS-autotest
+- crypto-autotest
+- nvme-phy-autotest
+- nvmf*-phy-autotest
+- vhost-autotest
+```
+
+* Ubuntu: Last two LTS releases. Currently `20.04` and `18.04`.
+
+```list
+- ubuntu18-vg-autotest
+- ubuntu20-vg-autotest
+```
+
+* CentOS: Maintained releases. Currently `7.9` and `8.2`.
+
+```list
+- centos7-vg-autotest
+- centos8-vg-autotest
+```
+
+## FreeBSD distributions {#freebsd_list}
+
+* FreeBSD: Production release. Currently `12.2`.
+
+```list
+- freebsd-vg-autotest
+```