Move documentation from wiki/gh-pages into README

[Fivell]

Summary

Consolidates the still-relevant snippets from the wiki and the gh-pages site into the README, and drops the links to those external sources. Both the wiki and the gh-pages site have been stale for years, so docs that live next to the code are easier for users to find and easier for us to keep current.

What's added to the README

• Authorization — minimal CanCanCan example (was in Home.md)
• Per-request context — short example of active_admin_import_context (the convention introduced in Add active_admin_import_context convention for controller-provided context #211 — was previously undocumented in the README)
• Examples section consolidating the wiki pages, each as a small focused snippet:
  • Files without CSV headers
  • Auto-detect file encoding
  • Force a specific (non-UTF-8) encoding
  • Disallow ZIP upload
  • Skip CSV columns
  • Resolve associations on the fly
  • Update existing records by id (with a note pointing at :on_duplicate_key_update for adapters that support upserts)
  • Tune batch size
  • Import into an intermediate table
  • Allow user input for CSV options (custom template + ERB)
  • Inspecting the importer in batch callbacks

What's removed

• The intro line pointing at activeadmin-plugins.github.io/active_admin_import/ and the wiki
• The trailing #### Wiki section that linked back to the same wiki

What's not changed

• The badges, Installation, Features, Basic usage, Options table, Custom ImportResult, Dependencies, and Contributing sections are all left alone
• No code changes — README only

Test plan

• Reviewed README rendering manually
• All migrated snippets reflect the current public API (verified against lib/active_admin_import/dsl.rb, options.rb, and model.rb)
• Stale fetch_extra_options_from_params snippet from the wiki Home page was dropped (the option no longer exists in the codebase)

Follow-up

Once this is merged, the wiki and gh-pages site can either be deleted or have a notice added pointing readers at the README.