Clarify boundary Franklin/KaTeX

[tlienart]

There's been a bit of confusion e.g. recently in #624. Basically

• $$ XXX $$ the entire content of XXX is passed un-touched to KaTeX
• \begin{EEE} XXX \end{EEE} the entire content of XXX is passed un-touched to KaTeX inside an environment managed by Franklin
  • EEE is align --> KaTeX effectively sees $$\begin{aligned} XXX \end{aligned}$$ (https://github.com/tlienart/Franklin.jl/blob/4f7ff3177cba1be5309f73c05f9fd7e94fe534d5/src/converter/markdown/blocks.jl#L62)
  • EEE is eqnarray --> KaTeX effectively sees $$\begin{array}{c} XXX \end{array}$$ (this should be tested btw...) https://github.com/tlienart/Franklin.jl/blob/4f7ff3177cba1be5309f73c05f9fd7e94fe534d5/src/converter/markdown/blocks.jl#L63

[tlienart]

yeah it should be \begin{array}{rcl} for EQA.