[manpages] Rewrite man pages in a simpler text format.

Makefile still needs work, but I wanted to commit to save progress.

man8/thin_trim.txt

@@ -0,0 +1,23 @@
+NAME
+  thin_trim - Issue discard requests for free pool space (offline tool).
+
+SYNOPSIS
+  thin_trim [options] --metadata-dev {device|file} --data-dev {device|file}
+
+DESCRIPTION
+  thin_trim sends discard requests to the pool device for unprovisioned areas.
+
+  This tool cannot be run on live metadata.
+
+OPTIONS
+  -h, --help	Print help and exit.
+  -V, --version	Print version information and exit.
+  --pool-inactive	Indicates you are aware the pool should be inactive.
+
+    Suppresses a warning message and prompt.
+
+SEE ALSO
+  thin_dump(8), thin_repair(8), thin_restore(8), thin_rmap(8), thin_metadata_size(8)
+
+AUTHOR
+  Joe Thornber <ejt@redhat.com>