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-hints.7.adoc

@@ -0,0 +1,54 @@
+= timew-hints(7)
+
+== NAME
+timew-hints - Timewarrior hints
+
+== SYNOPSIS
+
+== DESCRIPTION
+Timewarrior supports hints, which are single-word command line features that start with a colon like this:
+
+  :week
+
+Hints serve several purposes.
+This example is a shortcut for the date range that defines the current week.
+
+Other hints, such as:
+
+  :quiet
+
+Are ways to control the behavior of Timewarrior, in this case eliminating all forms of feedback, for purposes of automation.
+
+The supported hints are:
+
+  :quiet         Turns off all feedback. For automation
+  :debug         Runs in debug mode, shows many runtime details
+  :yes           Overrides confirmation by answering 'yes' to the questions
+
+  :color         Force color on, even if not connected to a TTY
+  :nocolor       Force color off, even if connected to a TTY
+  :blank         Leaves tracked time out of a report
+  :fill          Expand time to fill surrounding available gap
+  :adjust        Automatically correct overlaps
+  :ids           Displays interval ID numbers in the summary report
+
+Range hints provide convenient shortcuts to date ranges:
+
+  :yesterday     The 24 hours of the previous day
+  :day           The 24 hours of the current day
+  :week          This week
+  :fortnight     This week and the one before
+  :month         This month
+  :quarter       This quarter
+  :year          This year
+  :lastweek      Last week
+  :lastmonth     Last month
+  :lastquarter   Last quarter
+  :lastyear      Last year
+  :monday        Previous monday
+  :tuesday       Previous tuesday
+  :wednesday     Previous wednesday
+  :thursday      Previous thursday
+  :friday        Previous friday
+  :saturday      Previous saturday
+  :sunday        Previous sunday