documentation: add linux package instructions

[tjhei]

it is easier and faster to install on Ubuntu using the apt packages as described in https://github.com/dealii/dealii/wiki/Getting-deal.II#linux-packages
We should offer this as a fourth option in the manual.

[tjhei]

@gassmoeller : Would you add a fourth option to the table in https://aspect-documentation.readthedocs.io/en/latest/user/install/index.html corresponding to installing ASPECT from source but binaries from Ubuntu Packages? Otherwise, we could also put this into "compile & install" as an alternative to https://aspect-documentation.readthedocs.io/en/latest/user/install/local-installation/using-candi.html

[tjhei]

I guess there are the following options:

1. VM
2. docker
3. candi + ASPECT
4. Ubuntu deal.II + ASPECT
5. Ubuntu dependencies of deal.II + deal.II manually + ASPECT

Of these options only 3 and 5 give good GMG performance due to native optimizations.