Split 'User documentation' into basic and advanced category

[1-DimensionalBox]

I think it would make sense to split the user documentation into basic pages for users who want to get started, and advanced pages for experienced users who want to delve deeper into Freeplane's possibilities.

Basic

  * [First steps](first-steps.md)
  * [Working with mindmap files](working-w-mindmap-files.md)
  * [Formatting maps and nodes](formatting-maps-and-nodes.md)
  * [Spell checker](Spell_checker.md)

Advanced

  * [Styles](styles.md)
  * [Templates](templates.md)
  * [Conditional node styles](Conditional_node_styles.md)
  * [Extend with add-ons] <!-- See also issue #54 -->  
  * [CSS for HTML/Markdown](css-for-html-or-markdown.md)
  * [LaTeX in Freeplane](LaTeX_in_Freeplane.md)

Miscellaneous

  * [Glossary] <!-- See also issue #52 -->
  * [Relationship to FreeMind](Relationship_to_FreeMind.md)
  * [References to other known documentation/tutorial sites](other-docs-tutorials.md)

[1-DimensionalBox]

Excuse me for leaving this as an issue, but I'm not sure on how to move/rename files in docsify without making a mess. I will look into that.