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-config.7.adoc

@@ -0,0 +1,51 @@
+= timew-config(7)
+
+== NAME
+timew-config - Timewarrior configuration file and override options
+
+== SYNOPSIS
+**timew rc.**__<name>__**=**__<value>__ _<command>_
+
+== DESCRIPTION
+Timewarrior stores its configuration in the user's home directory in _~/.timewarrior/timewarrior.cfg_.
+This file contains a mix of rules and configuration settings.
+Note that the TIMEWARRIORDB environment variable can be set to override this location.
+
+The values 'true', '1', 'y', 'yes' and 'on' are all equivalent and enable a setting.
+Any other value means disable the setting.
+
+Default values may be overridden by timewarrior.cfg values, which may in turn be overridden on the command line using: **rc.**__<name>__**=**__<value>__
+
+For example, to turn off verbose mode:
+
+    rc.verbose=0
+
+Note that hints can also do this (:quiet).
+
+== CONFIGURATION
+
+*confirmation*::
+Determines whether harmful operations require interactive confirmation.
++
+May be overridden by the ':yes' hint.
++
+Default value is 'yes'.
+
+*verbose*::
+Determines whether Timewarrior generates feedback.
++
+May be overridden by the ':quiet' hint.
++
+Default value is 'yes'.
+
+*debug*::
+Determines whether diagnostic debugging information is shown.
++
+Useful for troubleshooting, but not for general use.
++
+Default value is 'off'.
+
+*debug.indicator*::
+The debug output prefix string.
++
+Default value is '>>'.