Do not use doxygen formulas `@f` in README

[marcfehling]

Blunt LaTeX syntax will be displayed correctly in both GitHub Readme and Doxygen. @f is only displayed nicely in Doxygen.

We should choose the format which works nicely in both.

I will upload an example here after #147 gets merged.

[marcfehling]

Swift_Hohenberg-Solver

$$\begin{aligned}
    \frac{\partial u}{\partial t} = ru - (k_c + \Delta)^2 u + g_1 u^2 - u^3
\end{aligned}$$

• GitHub: https://github.com/dealii/code-gallery/blob/master/Swift-Hohenberg-Solver/README.md
  
• Doxygen: https://www.dealii.org/developer/doxygen/deal.II/code_gallery_Swift_Hohenberg-Solver.html
  

Distributed_LDG_Method:

@f{align}
- \nabla \cdot  \left(\ \nabla u \ \right)&= f(\textbf{x}) && \mbox{in} \
\Omega,\nonumber \\
-\nabla u \cdot \textbf{n}  &= g_{N}(\textbf{x}) && \mbox{on} \ \partial 
\Omega_{N} \nonumber\\
u &= g_{D}(\textbf{x}) && \mbox{on}  \ \partial \Omega_{D}. \nonumber
@f}

• GitHub: https://github.com/dealii/code-gallery/blob/master/Distributed_LDG_Method/README.md
  
• Doxygen: https://www.dealii.org/developer/doxygen/deal.II/code_gallery_Distributed_LDG_Method.html
  

[bangerth]

#149 replaces the $$\begin{aligned} thing to proper doxygen markup. I would say that doxygen input is the canonical style. If we can make it work for markdown, that's nice, but that's not high on my list of priorities.