Add mission/vision/values and governance model documents

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Summary

Add two foundational governance documents:

• mission-vision-values.adoc: Establish project mission, vision, and core values that underpin all governance decisions
• governance-model.adoc: Explain how authority flows from Core Team through delegated teams to individual committers

Reorganize internal/_index.adoc and policies.adoc to reflect the
new governance structure with clear sections for governance,
teams, policies, and procedures. Both index pages now serve as
entry points with better hierarchy and cross-references.

Co-Authored-By: Claude Haiku 4.5 <[e-mail]>

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website/content/en/internal/mission-vision-values.adoc

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Note: These have been synthesized from our other docs.

The Mission and Vision likely need some quality time to hone.

The Values are better, but also need some work.