Add examples to prompt

What does this MR do and why?

Add examples to prompt to improve results.

rake task (please take with the grain of salt): improvement from 59% to 77% rspec: only one failure, which is actually a false negative:

 1) Gitlab::Llm::Chain::Agents::ZeroShot::Executor real requests with predefined issue input_template: "How old is %<issue_identifier>s issue?", tools: ["IssueIdentifier", "Resource Reader"], answer_match: /2 days/ answers query using expected tools
     Failure/Error: expect(answer.content).to match_llm_answer(answer_match)
       expected a string matching (?-mix:2 days) regexp, got 'The issue group1/project-1#1 was created on June 28, 2023.'

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