docs: Refresh build and installation guides

The top-level guides still describe retired source, QLogic, service and
kernel-support workflows. Following them can select the wrong driver,
cross-compile only part of the tree, or misstate module-signing policy.

Commit 7aade0a109 ("Makefile: Make the qla2x00t-32gbit driver
the default QLogic FC driver") made the newer tree the default.
Commit 0d3c9018af ("debian, scstadmin: Add systemd scst.service")
added the installed systemd unit.

Point source users at GitHub and kernel support at
nightly/conf/nightly.conf. Separate kernel and user-space cross builds,
follow the current service recipes, and make module acceptance depend
on the target kernel and platform policy.

README.md

@@ -14,8 +14,8 @@ project includes:
 1. The SCST core in the scst/ subdirectory.
 2. A tool for loading, saving and modifying the SCST configuration in
    directory scstadmin/.
-3. Several SCSI target drivers in the directories iscsi-scst/, qla2x00t/,
-   srpt/, scst_local/ and fcst/.
+3. Several SCSI target drivers in the directories iscsi-scst/,
+   qla2x00t-32gbit/, qla2x00t/, srpt/, scst_local/ and fcst/.
 4. User space programs in the usr/ subdirectory, e.g. fileio_tgt.
 5. Various documentation in the doc/ subdirectory.
 
@@ -26,8 +26,8 @@ file.
 
 Two QLogic target drivers are included in the SCST project.
 
-The default driver is located in qla2x00t-32gbit directory and it supports up
-to 32 Gb/s FC. It is the newer one.
+The default driver is located in the qla2x00t-32gbit directory. It is the
+newer one.
 
 May anyone wish to switch back to the older driver that only supported up to
 16 Gb/s adapters, it is located in qla2x00t directory. To make use of the