This commit extracts the information about the configuration the user should do
right after installation (especially running scylla_setup) to a separate file.
The file is included in the relevant pages, i.e., installing with packages
and installing with Web Installer.
In addition, the examples on the Web Installer page are updated
with supported versions of ScyllaDB.
Fixes https://github.com/scylladb/scylladb/issues/19908
(cherry picked from commit 849856b964)
Closes scylladb/scylladb#20050