Base: Add 3DFileViewer, CatDog & CertificateSettings manpages

Also update Applications.md to reflect this and some other previously
missing pages.

Base/usr/share/man/man1/Applications/CatDog.md

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+## Name
+
+![Icon](/res/icons/16x16/app-catdog.png) CatDog - A helpful companion
+
+[Open](file:///bin/CatDog)
+
+## Synopsis
+
+```**sh
+$ CatDog
+```
+
+## Description
+
+`CatDog` is an animated screenmate application in the form of a virtual pet.
+
+It was inspired by [Neko](https://en.wikipedia.org/wiki/Neko_(software)). The name derives from its ambiguous appearance: is it a cat, a dog, or a hybrid?
+
+Its helpful suggestions evoke the animated assistants popular at the turn of the millennium, such as [Clippy](https://en.wikipedia.org/wiki/Office_Assistant).
+
+Alongside [Buggie](https://en.wikipedia.org/wiki/SerenityOS#History) and the [Yaks](http://yaksplained.org/), CatDog is a much loved member of the SerenityOS family of mascots. As such it has a wider presence in the project’s online community, promotional materials and merchandise.
+
+### Features
+
+By default, CatDog will chase your mouse cursor around the screen. It will always stay on-top of other windows.
+
+Additionally, CatDog offers helpful suggestions depending on the currently active application.
+
+CatDog has multiple states:
+
+- *Sleeping* if the mouse cursor hasn’t moved in some time
+- *Alert* if woken by the mouse cursor or speaking after being idle
+- *Dressed as an Artist* if Pixel Paint or [Font Editor](help://man/1/Applications/FontEditor) are open
+- *Dressed as an Inspector* if [Profiler](help://man/1/Applications/Profiler) or System Monitor are open
+
+To exit, right-click on CatDog and select `Quit` or, if active, press `Alt+F4`.
+
+CatDog brings good luck to those who keep it open.