Update Docstrings in all classes related to RandomVariables

[JonathanWenger]

This PR updates and adds missing docstrings to the recently refactored classes associated with RandomVariable.

Docstrings

• Update the documentation thoroughly in all changed files
• define what we mean by the "distribution of a random variable"
• Warning about order of operations in RandomVariable (closes Write a warning into RandomVariable about order of operations #146)
• make the analogy to numpy arrays explicit in the documentation

Tutorials and Quickstart Guide:

• Update the quickstart guide and notebooks explaining the use of random variables

Developer Guides:

• [ ] describe the steps to add a new RandomVariable subclass, including how to define new arithmetic operations

Other:

• alter the __repr__ of RandomVariables (see also Documentation of changes in RandomVariable #199 (comment))
• document that we only want to use np.generic and np.ndarray internally, if possible

This PR closes #199.

Note: You can view the built documentation by going to the "Details" page of the docs/readthedocs.org:probnum check, which builds a version of the documentation corresponding to this PR.

[codecov]

Codecov Report

Merging #213 (38f12ba) into master (426199a) will increase coverage by 0.48%.
The diff coverage is 86.48%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   74.40%   74.88%   +0.48%     
==========================================
  Files          60       60              
  Lines        3169     3158      -11     
  Branches      414      410       -4     
==========================================
+ Hits         2358     2365       +7     
+ Misses        647      624      -23     
- Partials      164      169       +5     

Impacted Files	Coverage Δ
src/probnum/__init__.py	77.77% <ø> (ø)
src/probnum/diffeq/__init__.py	100.00% <ø> (ø)
src/probnum/diffeq/ode/ivp.py	98.57% <ø> (ø)
src/probnum/diffeq/odefiltsmooth/ivp2filter.py	81.14% <0.00%> (ø)
src/probnum/diffeq/odefiltsmooth/odefiltsmooth.py	68.68% <ø> (ø)
src/probnum/diffeq/odesolution.py	97.14% <ø> (ø)
src/probnum/filtsmooth/__init__.py	100.00% <ø> (ø)
src/probnum/linalg/__init__.py	100.00% <ø> (ø)
src/probnum/linalg/linearsolvers/linearsolvers.py	47.61% <ø> (ø)
src/probnum/linalg/linearsolvers/matrixbased.py	58.35% <ø> (ø)
... and 16 more

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[marvinpfoertner]

I added a comment every time something didn't read smoothly, so my comments are relatively noisy. Not everything needs fixing.