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-get.1.adoc

@@ -0,0 +1,22 @@
+= timew-get(1)
+
+== NAME
+timew-get - display DOM values
+
+== SYNOPSIS
+[verse]
+*timew get* _<DOM>_**...**
+
+== DESCRIPTION
+Validates the DOM reference, then obtains the value and displays it.
+
+== EXAMPLES
+For example:
+
+    $ timew get dom.active
+    1
+
+It is an error to reference an interval or tag that does not exist.
+
+== SEE ALSO
+*timew-DOM*