doc: migrate wiki to docsify page

[skaldarnar]

Similar to Terasology/Health#74 this PR moves the content of the wiki into the docs folder to be served as gh-pages powerd by docsify. This allows to edit and update the documentation alongside the code more easily.

I moved symlinked the README file to the docs folder, which works with both Github's web view and later in docsify as well. See https://github.com/skaldarnar/Behaviors. (The image is not rendered when viewing the symlinked file in Github, but we can probably accept that.)

However, I'm not sure of the interop with Windows checkouts of the repo :thinking: Looking at Git symbolic links in Windows on StackOverflow this seems to be a bigger issue I don't want to solve right now. The symlink can work under Windows if the respective setting is enabled when installing Git Bash for Windows, for instance. If not, the symlink is replaced with a file containing a reference to the target (which does not render perfectly, but also does not look like it will cause edit wars by accident).

A preview of the github pages view can be seen at https://skaldarnar.github.io/Behaviors/#/

• [x] decide how to proceed with symlinks

Not blocking the PR from my view, but would be nice to have:

• [x] update the banner image
• [x] update the icon image

[skaldarnar]

Potential Follow-ups

• "quick start" and "big picture" share the same content - merge them?
• use same notation for "things", e.g.,
  • Node vs node vs node
• use the same wording
  • Control Flow Nodes, Action Nodes
  • Actions, Decorators
• explain naming of Control Flow Nodes (or Nodes in general?)
• template to explain nodes
  • name
  • description
  • parameters
  • result values
  • example
• "special nodes" is a bad name - find a better name that explains what those nodes are/do
• "Actions and Decorators" is about how to implement them
• "Control Flow Nodes" explains how to use them
• incorporate https://github.com/terasology/tutorialgroups to the content