Add documentation menu

[snipercup]

This adds a menu to read documentation in-game. It can be accessed from the main menu. It is intended to be used by developers and players. The players will access the gameplay section of the documentation for help. Developers will access the modding section of the documentation. It is desigend to read .md files so the help files can be read on github as well.

The limitation this creates is that only markdown that is supported in this addon can be used. If you add other markdown then is supported in the addon, it will probably render fine on Github but not in-game. If additional formatting is needed, you can always contribute to that repository so everyone can use new functionality.

It can already read documentation files (not images) that were not shipped with the game from your local disk. This requires that the Documentation folder is present in the game directory so you would have to download that from the repository.

This is what it looks like in-game:

This PR introduces the basic version of the documentation reader but additional functionality could be added in the future:

1. Ability to read images from the local disk that were not shipped with the game (for mods). This requires dynamic resource loading which will be useful for other modding applications as well
2. An in-game documentation editor
3. Mod support so any mod can have its own documentation directory