Move capabilities.html back to the main directory

[jidanni]

https://www.gpsbabel.org/htmldoc-development/Data_Filters.html has a link to
https://www.gpsbabel.org/capabilities.html , but:

• That means there is no link that can be used offline, e.g., in
  Debian, to see the capabilities of the version we have right there
  locally on our disk.

So you ought to move capabilities.html back to the main directory.

And if users want to see some future version, they would check
htmldoc-development/capabilities.html
which would have the usual "Note: This page describes the development
version of GPSBabel..." instead of "This list is updated more frequently
than we release GPSBabel itself. It will thus include formats and
filters that are present only in beta versions or source builds."

[robertlipe]

There are no plans to change web design to accomodate Debian packaging for offline use. Sorry.