Rebrand: naming guidance for agents and contributors #1025

Part of #1024. Step 1, do this first so every other change follows the same rule.

Add the naming rule (see the parent issue) where people and agents look before writing.

• .github/copilot-instructions.md: add a short "Product name" section with the rule. Also fix the outdated intro line "VS Code Extension for Azure Cosmos DB and the MongoDB API".
• CONTRIBUTING.md: title and the 4 mentions in the body; add the rule for contributors.
• .github/skills/writing-release-notes/ (SKILL.md, RELEASE-NOTES-FORMAT.md, EXAMPLES-MAJOR-RELEASE.md): templates use "DocumentDB for VS Code Extension vX.Y"; switch to the new name in the template and title format. Keep quoted historical examples as they are, or mark them as historical.
• .github/skills/documentation-screenshots/ (SKILL.md, scripts/frame.py docstring).
• Skill descriptions that say "DocumentDB VS Code extension" or "vscode-documentdb extension" (react-webview-architecture, telemetry-instrumentation) and src/services/taskService/README.md, src/tree/README.md: low priority, internal.