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.adoc

@@ -0,0 +1,31 @@
+= timew-start(1)
+
+== NAME
+timew-start - start time tracking
+
+== SYNOPSIS
+[verse]
+*timew start* [_<date>_] [_<tag>_**...**]
+
+== 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.
+
+== EXAMPLES
+For example, this command specifies two tags ('weekend' and 'Home & Garden'), the second of which requires quotes.
+
+    $ timew start weekend 'Home & Garden'
+
+An optional date may be specified to indicate the intended start of the tracked time:
+
+    $ timew start 8am weekend 'Home & Garden'
+
+If there is a previous open interval, it will be closed at the given start time.
+
+Quotes are harmless if used unnecessarily.
+
+== SEE ALSO
+**timew-cancel**(1),
+**timew-continue**(1),
+**timew-stop**(1),
+**timew-track**(1)