docs: unify YAML code block indentation

[sisp]

I've unified the indentation of YAML code blocks in the docs to 4 spaces per indentation level.

I think most projects use 4 spaces for Python code and 2 spaces for most other languages like YAML. However, Prettier currently cannot be configured accordingly because it doesn't support config overrides by langauge but only by file (pattern). Markdown indentation was changed to 4 spaces in #249 for good reasons, but as a result also e.g. YAML code blocks are formatted with 4 spaces indentation. I believe the reason why the two occurrences of 2 spaces indentation, that I'm changing to 4 spaces in this PR, weren't caught by Prettier is a result of the non-standard Markdown syntax used for admonitions that isn't recognized as such by Prettier (or rather by the underlying Markdown parser) but instead, as far as I can tell, as indented code blocks, so that a fenced code block inside an admonition is treated as a code block inside a code block, which Prettier won't touch.

This PR also fixes a "regression" introduced in #800.

[codecov-commenter]

Codecov Report

Merging #808 (e2ede6c) into master (608654a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #808   +/-   ##
=======================================
  Coverage   96.67%   96.67%           
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  Files          41       41           
  Lines        2951     2951           
=======================================
  Hits         2853     2853           
  Misses         98       98           

Flag	Coverage Δ
unittests	96.67% <ø> (ø)

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