archive docs in CI

[tsteven4]

change cmake variable names to conform to cmake standards WEB -> GPSBABEL_WEB DOCVERSION -> GPSBABEL_DOCVERSION

add a configurable CI job, gendocs, to generate docs and archive them. This can be manually triggered to run on any branch/tag using a specified value for DOCVERSION. It will also be run on pushes to master and a schedule.

[robertlipe]

Good timing. Just last night I was struggling with:

• xsltproc --stringparam base.dir gpsbabel.org/htmldoc-x.y.z/ --stringparam root.filename index xmldoc/babelmain.xsl xmldoc/readme.xml error : Unknown IO error warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl-ns/current/xhtml/chunk.xsl" compilation error: file xmldoc/babelmain.xsl line 7 element import xsl:import : unable to load http://docbook.sourceforge.net/release/xsl-ns/current/xhtml/chunk.xsl

I'm increasingly forgetting how to even build this stuff. :-/

[tsteven4]

I plan to include the docs (gpsbabel.pdf, gpsbabel.html, gpsbabel.org) in the draft release along with the installers.