Replace roff man pages with asciidoctor

This replaces the generation of man pages on project setup
by a on-demand generation via asciidoctor.
An exception are the man pages for the commands `day`, `month`, and `week`
which are simply redirects to the man page `timew-chart.1`. Those are now
static files in the Timewarrior repository.

A CMake find module to detect asciidoctor was added.
If asciidoctor is found, the targets `doc`, `man1`, and `man7` are created.
Those targets are also added to the default build target.

If asciidoctor is not available, the target `doc` is available, but it only
emits a message to install asciidoctor first.

Signed-off-by: Thomas Lauf <thomas.lauf@tngtech.com>

doc/man1/timew-stop.1.adoc

@@ -0,0 +1,32 @@
+= timew-stop(1)
+
+== NAME
+timew-stop - stop time tracking
+
+== SYNOPSIS
+[verse]
+*timew stop* [_<date>_] [_<tag>_**...**]
+
+== DESCRIPTION
+Stops tracking time.
+If tags are specified, then they are no longer tracked.
+If no tags are specified, all tracking stops.
+
+== EXAMPLES
+For example:
+
+    $ timew start tag1 tag2
+    ...
+    $ timew stop tag1
+
+Initially time is tracked for both 'tag1' and 'tag2', then 'tag1' tracking is stopped, leaving tag2 active.
+To stop all tracking:
+
+    $ timew stop
+
+
+== SEE ALSO
+**timew-cancel**(1),
+**timew-continue**(1),
+**timew-start**(1),
+**timew-track**(1)