Relocate default palette table to own Appendix

[dansanderson]

The only description of the default system palette is currently in the BACKGROUND entry of the BASIC keyword reference. Several places refer to this and currently have to just refer to the BACKGROUND entry (colourtable). It'd be a small improvement to give this table its own short appendix and have everyone refer to the appendix.

[dansanderson]

This is done.