Document Percona Server for MySQL versioning scheme

Percona Server for MySQL users should be aware of what each number in the product version specification (<X>.<Y>.<Z>-<N>) mean.
For instance, in 8.0.30-22:

• 8.0.30 - the version of the Oracle's MySQL Server, the Percona Server for MySQL is based on

• 22 - the sequential number of the Percona product release, increased every time Percona releases a new version of this particular product within its major branch (5.7 or 8.0 in this case)

Please notice, that Percona Server for MySQL may have several releases based on the same Oracle MySQL Server version.
For instance, 8.0.28-19 and 8.0.28-20.