[doxygen] add description for "exceptions" group

Documentation tweaks

This is cosmetic change, made mostly to simplify copy-pasting from
documentation.

CI: minor updates

Add gdbserver support to ctest

it is supposed to be a replacement for CentOS, which will be EOL soon,
(potentially along with RockyLinux, in the future).

Merge stable-1.13 into master

docker: add missing hub package

docs: update miscellaneous Doxygen comments

it wasn't added into Fedora 34, while we bumped 'doc' job to use latest Fedora.

docs: update and extend containers information

CI: use Fedora 34 instead of 32, since 32 is EOL now

as long as it's working it can run in nightly build.

Merge stable-1.12 into stable-1.13

Fix Docker image version(s) in CI

CI: update valgrind to 3.17

It's copy pasted from pmemkv (0d764bd).
Without this fix doc job may fail if image changed for master.

This solution is copy-pasted from pmemkv (70cd9e4).
Without this change PRs created against any stable branch may end up failing,
because it uses 'latest' image, instead of proper stable (e.g. '1.13') image.

We now use x.y.z format for all releases and the regex did not work well
with 1.13.0 release.

our pmemcheck tool was recently rebased to upstream 3.17 version.
It contains at least few useful bug fixes, e.g.:
https://bugs.kde.org/show_bug.cgi?id=426144
https://bugs.kde.org/show_bug.cgi?id=407764

Avoid libunwind on valgrind tests

docs: fix docstring

Initialize atomics in ringbuffer

It's less error prone to initialize atomics with default value, than not
initializing and calling store() in every constructor.

[doc] add description for Transaction API feature

This commit provides user introduction to transaction world.

tests: add more TCs for pool