Deprecate google-genai and vertexai instrumentations, deprecate the genai folder

[lmolkova]

• update docs for google-genai and vertex
• update instructions, contrib, agents.md to point to new repo for all genai instrumentations
• clean up remaining mentions of genai instrumentations

[lmolkova]

these are released from the new repo, removing from here

[Copilot]

Pull request overview

This PR updates repository documentation and contributor guidance to reflect that GenAI instrumentations have moved to the separate opentelemetry-python-genai repository, and removes/updates remaining references to developing or releasing GenAI packages from this repo.

Changes:

• Add deprecation/migration warnings and update links for GenAI instrumentations (notably Google GenAI and Vertex AI).
• Update contributor/agent instructions and CONTRIBUTING guidance to point GenAI work to the new repository.
• Remove GenAI packages from release workflow choices and remove a GenAI editable-install example from docs.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
instrumentation-genai/README.md	Updates top-level warning and points google-genai row at the new repo.
instrumentation-genai/opentelemetry-instrumentation-vertexai/CHANGELOG.md	Adds a deprecation warning explaining Vertex AI SDK deprecation context.
instrumentation-genai/opentelemetry-instrumentation-google-genai/README.rst	Replaces migration note with a stronger deprecation/migration warning and upgrade guidance.
instrumentation-genai/opentelemetry-instrumentation-google-genai/CHANGELOG.md	Adds a deprecation warning and removes the “Unreleased” section.
instrumentation-genai/AGENTS.md	Replaces detailed GenAI contribution rules with a pointer to the new repo.
docs/index.rst	Removes the editable-install example line for a GenAI package.
CONTRIBUTING.md	Updates GenAI guidance to direct contributions to the new repo and adjusts TOC/section structure.
AGENTS.md	Marks instrumentation-genai/ as deprecated and links to the local GenAI AGENTS guidance.
.github/workflows/package-prepare-release.yml	Removes GenAI packages from the manual “prepare release” workflow options list.
.github/instructions/instrumentation.instructions.md	Adds explicit “GenAI maintained elsewhere” guidance and renumbers subsequent sections.
.github/instructions/instrumentation-genai.instructions.md	Replaces detailed GenAI-specific review rules with deprecation guidance and redirect instructions.