User manual: reorganize text on importing from other dive log software

Change and shorten in some places the section dealing with data import
from foreign software.

Create Appendix C, containing information on data export from foreign
packages.

Signed-off-by: Willem Ferguson <willem@willem-Precision-M4700.(none)>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: update html file shipped with git repository

Every once in a while I need to update this so people building from source
who don't have asciidoc installed still have a reasonably current manual.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>