Update README to match the current code

[LeMyst]

Summary

• Complete the table of contents (fast run subsections, new "Other helpers" section)
• Fix the search_entities() description (real parameters, default max_results 50) and add an example
• Fix merge_items(): ignore_conflicts is a list, not a boolean; drop obsolete wbi_core mentions
• Datatypes: 18 types, add EntitySchema, fix the wikibaseintegrator.datatypes namespace
• OAuth impersonation section refers to wbi_login.OAuth1 instead of wbi_login.Login
• Fix the references snippet (new_claim_string vs claim_string)
• Mention Python 3.15 preview, drop the Python 2 / administrator rights notes
• Document the Poetry docs/notebooks groups and the offline / integration test commands
• List the other wbi_helpers functions (merge_lexemes, remove_claims, check_constraints, fulltext_search, delete_page, download_entity_ttl)

Test plan

• Check the Markdown rendering and the table of contents anchors on GitHub

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