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-start.1.in

@@ -1,36 +0,0 @@
-.TH timew-start 1 "${PACKAGE_DATE}" "${PACKAGE_STRING}" "User Manuals"
-.
-.SH NAME
-timew-start \- start time tracking
-.
-.SH SYNOPSIS
-.B timew start
-[
-.I <date>
-] [
-.I <tag>
-.B ...
-]
-.
-.SH DESCRIPTION
-Begins tracking using the current time with any specified set of tags.
-If a tag contains multiple words, therefore containing spaces, use quotes to surround the whole tag.
-.
-.SH EXAMPLES
-For example, this command specifies two tags ('weekend' and 'Home & Garden'), the second of which requires quotes.
-.RS
-$ timew start weekend 'Home & Garden'
-.RE
-An optional date may be specified to indicate the intended start of the tracked time:
-.RS
-$ timew start 8am weekend 'Home & Garden'
-.RE
-If there is a previous open interval, it will be closed at the given start time.
-.
-Quotes are harmless if used unnecessarily.
-.
-.SH "SEE ALSO"
-.BR timew-cancel (1),
-.BR timew-continue (1),
-.BR timew-stop (1),
-.BR timew-track (1)