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/man7/timew-dom.7.adoc

@@ -0,0 +1,29 @@
+= timew-dom(7)
+
+== NAME
+timew-dom - Timewarrior DOM
+
+== SYNOPSIS
+
+== DESCRIPTION
+Supported DOM references are:
+
+  dom.tag.count             Count of all tags
+  dom.tag.1                 Nth tag used
+
+  dom.active                '1' if there is active tracking, otherwise '0'
+  dom.active.tag.count      Count of active tags
+  dom.active.tag.1          Active Nth tag
+  dom.active.start          Active start timestamp (ISO Extended local date)
+  dom.active.duration       Active elapsed (ISO Period)
+  dom.active.json           Active interval as JSON
+
+  dom.tracked.count         Count of tracked intervals
+  dom.tracked.1.tag.count   Count of active tags
+  dom.tracked.1.tag.1       Tracked Nth, Nth tag
+  dom.tracked.1.start       Tracked Nth, start time
+  dom.tracked.1.end         Tracked Nth, end time, blank if closed
+  dom.tracked.1.duration    Tracked Nth, elapsed
+  dom.tracked.1.json        Tracked Nth, interval as JSON
+
+  dom.rc.<name>             Configuration setting