Add skills to help with instrumentation migration

[lmolkova]

• migrate-from-openinference — migrate an openinference-instrumentation-*
  package into this repo as an OTel GenAI package, or augment an existing
  package with the coverage OpenInference adds on top.
• review-migration — review a ported or augmented package against its
  upstream implementation and write MIGRATION_REPORT.md.
• write-conformance-tests — author conformance scenarios and the
  test_conformance.py runner for an instrumentation package.

[Copilot]

Pull request overview

This PR adds three repository “skills” under .github/skills/ to streamline common instrumentation migration workflows (migration from OpenInference, migration review reports, and authoring conformance tests), and documents their intended usage for contributors.

Changes:

• Documented available skills and when to trigger them in CONTRIBUTING.md.
• Added gitignore rule for generated MIGRATION_REPORT.md outputs.
• Introduced three new skills: migrate-from-openinference, review-migration, and write-conformance-tests.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
CONTRIBUTING.md	Adds a “Skills” section describing the new automation skills and when to use them.
.gitignore	Ignores generated MIGRATION_REPORT.md files.
.github/skills/write-conformance-tests/SKILL.md	New skill: guidance for writing GenAI conformance scenarios and the runner.
.github/skills/review-migration/SKILL.md	New skill: guidance for reviewing migrations and generating MIGRATION_REPORT.md.
.github/skills/migrate-from-openinference/SKILL.md	New skill: step-by-step workflow for migrating/augmenting OpenInference instrumentations into this repo.