update readme to be a bit less confusing/outdated

update readme to be a bit less confusing/outdated

E.g., pve-ha-manager is our current HA manager, so talking about the
"current HA stack" being EOL without mentioning the actually meant
`rgmanager` one, got taken up the wrong way by some potential users.
Correct that and a few other things, but as there are definitively
stuff still out-of-date, or will be in a few months, mention that
this is an older readme and refer to the HA reference docs at the
top.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>