docs: explain csi driver architecture and restructure docs

[lukasmetzner]

This is not yet a move to https://diataxis.fr/.

I mainly want to have some sort of indexing for the two documents how-to-import-volumes.md and csi-driver-architecture.md. This is currently hard to do nicely, as all our k8s docs are in one big README.md. For this reason, I moved all level two headings of the Kubernetes README.md into separate files. The content was not edited.

Additionally, I restructured the root README.md a bit to be more consistent with other k8s repositories.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.59%. Comparing base (ab67865) to head (eb42015).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           docs-explain-sidecars    #1268      +/-   ##
=========================================================
+ Coverage                  71.52%   71.59%   +0.06%     
=========================================================
  Files                         17       17              
  Lines                       1475     1475              
=========================================================
+ Hits                        1055     1056       +1     
+ Misses                       334      333       -1     
  Partials                      86       86              

Flag	Coverage Δ
e2e-controller	56.61% <ø> (+0.06%)	⬆️
e2e-node	56.61% <ø> (+0.06%)	⬆️
integration	3.31% <ø> (ø)
unit	40.94% <ø> (ø)

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[apricote]

Looks good besides the one comment. I did not verify if the content of the split up pages equals the old content.