docs: add missing content to global-tone-voice.mdx

[gokdeniz28]

This PR adds missing content to global-tone-voice.mdx, which is currently an empty file in the repository. An empty documentation file can lead to incomplete rendering and creates ambiguity for contributors regarding expected writing standards.

The added content introduces clear tone and voice guidelines to ensure consistency across documentation and improve the overall developer experience.

The change is intentionally minimal and scoped to a single file.
The content aligns with existing documentation principles found in other guideline files (e.g., content-instructions.md).
No structural or formatting changes were introduced beyond filling the empty file.

Verified that the MDX file renders correctly without syntax errors.
Confirmed that the added content follows the repository’s documentation style and formatting conventions.
Ensured no other files or parts of the documentation were affected.

[cb-heimdall]

🟡 Heimdall Review Status

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Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2