add common mistakes documentation

[isaacsas]

There are certain mistakes / misunderstandings that seem to commonly arise for newer users. We should add a common mistakes page and link it prominently from the tutorials.

Examples include

1. Using @species and @parameters outside of @reaction_network and not understanding that these are actually completely unconnected.
2. Similarly for defining t outside of @reaction_network.
3. Using interpolation when it isn't needed (or if it is needed not using it sufficiently in that interpolation of only expressions but not external variables/species ends up being used which is like point 1. above).