update building from source directions.

[tsteven4]

This resolves #1402.

@robertlipe I can generate the updated documents on github.

[robertlipe]

Thanx. For two words changed, I just edited Source.tpl on the server.

I'm just in no mood to debug xsltproc for something so trivial; it's not a battle worth winning.

Consider it done. Thanx.

[tsteven4]

The changes in this PR are a bit more extensive than 2 words, 2 paragraphs changed.

@robertlipe https://www.gpsbabel.org/htmldoc-1.10.0/Source.html and https://www.gpsbabel.org/htmldoc-development/Source.html both still refer to "Qt version 5.12", "qt5-qtbase-devel", "qt5", "qt5-default" as well as old package names for libusb. I also updated the related links.

We can easily generate the html document on github actions with the gendocs workflow. This works for any branch and docversion. It alleviates the need to maintain a document building environment on personal machines. I can help with this if it is useful.