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-gaps.1.adoc

@@ -0,0 +1,25 @@
+= timew-gaps(1)
+
+== NAME
+timew-gaps - display time tracking gaps
+
+== SYNOPSIS
+[verse]
+*timew gaps* [_<range>_] [_<tag>_**...**]
+
+== DESCRIPTION
+Displays a summary of time that is neither tracked nor excluded from tracking.
+
+The 'reports.gaps.range' configuration setting overrides the default date range.
+The ':blank' hint causes only the excluded time to be shown, with no tracked time.
+The default date range shown is ':day'.
+
+The ':blank' hint causes only the excluded time to be shown, with no tracked time.
+
+== CONFIGURATION
+**reports.gaps.range**::
+For reports that show a range of data, this setting will override the default value.
+The value should be a range hint, see **timew-hints**(7).
+
+== SEE ALSO
+**timew-summary**(1)