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-track.1.adoc

@@ -0,0 +1,27 @@
+= timew-track(1)
+
+== NAME
+timew-track - add intervals to the database
+
+== SYNOPSIS
+[verse]
+*timew track* _<range>_ [_<tag>_**...**]
+
+== DESCRIPTION
+The track command is used to add tracked time in the past.
+Perhaps you forgot to record time, or are just filling in old entries.
+
+== EXAMPLES
+For example:
+
+    $ timew track :yesterday 'Training Course'
+    $ timew track 9am - 11am 'Staff Meeting'
+
+Note that the track command expects a closed interval (start and end time), when recording.
+If a closed interval is not provided, the 'track' command behaves the same as the 'start' command.
+
+== SEE ALSO
+**timew-cancel**(1),
+**timew-continue**(1),
+**timew-start**(1),
+**timew-stop**(1)