This pull request adds dynamic substitutions for the following variables:
* `.. |CURRENT_VERSION| replace:: {current_version}`
* `.. |UBUNTU_SCYLLADB_LIST| replace:: scylla-{current_version}.list`
* `.. |CENTOS_SCYLLADB_REPO| replace:: scylla-{current_version}.repo`
As a result, it is no longer needed to update the "Installation on Linux" page manually after every new release.
Closesscylladb/scylladb#17544