docs: Removed summary ssh and pgp pages. Added about-ssh about-pgp and pub-priv pages.

This is the first set of commits related to refactoring the Seahorse help to make it easier to find
relevant topics. There's some added content, but it's largely just reorganizing the help.
docs: Updates to ssh-related topics

Refactored the help related to SSH tasks and concepts.
docs: Refactoring pgp-related topics

The third commit related to refactoring of the Seahorse help. Largely
changes related to documentation structure. Not much change in the way
of documentation content.
docs: Updates related to refactoring of the keyring topics

Fourth update to Seahorse documentation. Updates are largely related
to structure - not content.
docs: Last commit as related to the documentation refactoring.

There may be further tweaks to the help for the 3.16 release, but this
should be the final commit in the larger Seahorse help refactoring effort.