Add something like "Run your first simulation" to docs

[pmocz]

Really cool, performance-portable code with unique physics features! I'm excited to see it grow :)

I'd find it very helpful to have a "Run your first simulation" page under "Getting started" that rapidly goes through an example of running your first simulation (e.g. just locally on your laptop) and making a pretty picture -- something like the 2 stream instability or weibel -- preferably that takes just a few minutes to run. A lightening recipe without all the details. Sorry if this is already in the docs and I missed it!

e.g.

blah blah blah

git clone --recursive git@github.com:entity-toolkit/entity.git

blah blah

cmake -B build -D pgen=srpic/weibel -D output=ON

blah blah

cmake --build build -j 8

blah blah

build/src/entity.xc -input setups/srpic/weibel/weibel.toml 

...