Split symbols reference page into a guide and a reference page

[FichteFoll]

Currently, the Reference > Symbols page tries to achieve both purposes, starting with a rather short guide and then ending with a reference to explain the various options. It should be split in two so that the nitty gritty details can be explained on the reference page and the guide page can focus on explaining how it works and how to get it working for your syntax.

The reference part of symbols could also simply be added to the existing "Metadata" reference page (for tmPreferences) instead. This page could hold the full reference for all supported keys but the guide section would have separate pages for each concept (comments, indentation, symbols, folding).