[FLINK-40772][docs] Add a PR template with AI disclosure

[Savonitar]

Purpose of the change

The Training repository has no PR template. Add one so contributors can provide review context and disclose AI assistance as described in the Flink contribution guide.

Addresses FLINK-40772, part of FLINK-40749.

Brief change log

• Link the existing CONTRIBUTING.md and Flink contribution guides.
• Add concise prompts for purpose, exercise and solution changes, verification, learner-facing impact, and documentation.
• Include the current Flink AI-disclosure section, linking the contribution guide instead of an absent AGENTS.md.

Verifying this change

Checked the complete one-file diff, whitespace, Apache license header, HTML comment boundaries, UTF-8 encoding, LF line endings, and final newline. Confirmed that the AI checkbox and Generated-by placeholder remain visible outside comments. No Gradle build or runtime tests were run for this Markdown-only change.

Impact and documentation

Adds contribution guidance only. No changes to exercises, solutions, dependencies, or learner setup.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: OpenAI Codex (GPT-6)

[spuru9]

I dont think this PR is relevant. The repo is practically dead for 3 years.