Port documentation to gi-docgen

Docs can be previewed at https://msandova.pages.gitlab.gnome.org/libsecret/.

In gi-docgen pages are in correspondence with classes/structs/etc meaning that some doc strings like https://developer-old.gnome.org/libsecret/unstable/libsecret-SecretSchema.html are not displayed anywhere. All these floating docs were moved to Simple API and need a bit of work.

It would be nice to have a logo for the library to display on the repo and docs, it is possible to request one at https://gitlab.gnome.org/Teams/Design/library-icon.