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-summary.1.adoc

@@ -0,0 +1,33 @@
+= timew-summary(1)
+
+== NAME
+timew-summary - display a time-tracking summary
+
+== SYNOPSIS
+[verse]
+*timew summary* [_<range>_] [_<tag>_**...**]
+
+== DESCRIPTION
+Displays a report summarizing tracked and untracked time for the current day by default.
+Accepts date ranges and tags for filtering, or shortcut hints:
+
+    $ timew summary monday - today
+    $ timew summary :week
+    $ timew summary :month
+
+The ':ids' hint adds an 'ID' column to the summary report output for interval modification.
+
+== CONFIGURATION
+**reports.summary.holidays**::
+Determines whether relevant holidays are shown beneath the report.
+Default value is 'yes'.
+
+== SEE ALSO
+**timew-day**(1),
+**timew-lengthen**(1),
+**timew-modify**(1),
+**timew-month**(1),
+**timew-shorten**(1),
+**timew-tag**(1),
+**timew-untag**(1),
+**timew-week**(1)