docs: dt: writing-schema: mention coding style

Mention the usage of YAML coding style.  Describe explicitly that
four-space indentation in DTS examples is preferred, because:
1. The YAML's default two-space indentation for DTS code makes it
   significantly less readable.
2. Linux coding style tabs would introduce inconsistency (entire file is
   indented with spaces).
3. On the other hand, eight spaces would not align with example's
   opening '  - |' part.  Four spaces makes the code nicely aligned with
   it.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220403081849.8051-2-krzysztof.kozlowski@linaro.org