Update DOCUMENTATION_GUIDELINES.md (#14369)

* Update DOCUMENTATION_GUIDELINES.md

* update ToC

* Update docs/DOCUMENTATION_GUIDELINES.md

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update docs/DOCUMENTATION_GUIDELINES.md

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* fix

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Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: parity-processbot <>