docs: fix spelling, grammar, (image-)links; unify reccuring expressions

[AJPfleger]

I fixed mainly those things I stumbled over during my first days, since some of them caused a little confusion for me.

• spelling/grammar/typos
• missing images links: the relative links were pointing to the wrong layer
• updated link to Pythia8: they changed their website
• missing expressions in commands

I unified some expressions:

• DD4hep instead of DD4Hep
• and instead of <build_dir>, , , ...
• multiline cmake expressions for the terminal

[paulgessinger]

Sure, why not.

[codecov]

Codecov Report

Merging #1259 (50f9d46) into main (6ca91a3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1259   +/-   ##
=======================================
  Coverage   47.89%   47.89%           
=======================================
  Files         375      375           
  Lines       19588    19588           
  Branches     9214     9214           
=======================================
  Hits         9382     9382           
  Misses       3822     3822           
  Partials     6384     6384           

Impacted Files	Coverage Δ
Core/include/Acts/Definitions/Common.hpp	92.30% <ø> (ø)
Core/include/Acts/Propagator/EigenStepper.hpp	68.42% <ø> (ø)
.../include/Acts/Propagator/MultiEigenStepperLoop.hpp	70.37% <ø> (ø)
...re/include/Acts/Propagator/StraightLineStepper.hpp	67.85% <ø> (ø)
Core/include/Acts/TrackFitting/KalmanFitter.hpp	45.07% <ø> (ø)

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