Add app pages to docs

[garrett]

https://cockpit-project.org/blog/making-a-cockpit-application.html is under blog; it should be shown under the docs section. Anything developer related, like this, should be there on https://cockpit-project.org/documentation.html

[garrett]

Aha, it's linked under https://cockpit-project.org/external/wiki/Contributing.html — I wonder if we should rename "Contribute" to "Develop"?

[garrett]

Also, searching for metainfo.xml doesn't work, but metainfo is fine. I guess I should dig into my old search code.

[garrett]

Additionally, there's a generator @ https://www.freedesktop.org/software/appstream/metainfocreator/#/addon which might be useful for us too.

@mvollmer: Should we revisit this page together and freshen it up a little? (It's been a few years.)