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-split.1.adoc

@@ -0,0 +1,27 @@
+= timew-split(1)
+
+== NAME
+timew-split - split intervals
+
+== SYNOPSIS
+[verse]
+*timew split* _<id>_**...**
+
+== DESCRIPTION
+Ѕplits an interval into two equally sized adjacent intervals, having the same tags.
+Using the 'summary' command, and specifying the ':ids' hint shows interval IDs.
+Using the right ID, you can identify an interval to split.
+
+== EXAMPLES
+For example, show the IDs:
+
+    $ timew summary :week :ids
+
+Then having selected '@2' as the interval you wish to split:
+
+    $ timew split @2
+
+== SEE ALSO
+**timew-join**(1),
+**timew-lengthen**(1),
+**timew-shorten**(1)