docs: Fix inaccuracies, unify wording, and expand SDK documentation

[vdusek]

Reviews the SDK documentation (Introduction, Concepts, Upgrading) against the codebase — fixes verified inaccuracies, unifies wording, and fills a few gaps. Does not touch the Guides section; those changes live in #939.

Correctness fixes

• Storages: documented the real Dataset.export_to (content_type='csv'|'json'); export_to_csv/export_to_json don't exist.
• Upgrading to v2: Actor.start_task → Actor.call_task (accepts Webhook, returns ActorRun).
• Introduction: Python requirement 3.10+ → 3.11+.
• Actor events: corrected the SYSTEM_INFO / MIGRATING payloads and Config → Configuration, each event now links its real data type; fixed 04_actor_events.py (passing a Pydantic model as logging extra= raises TypeError).
• Configuration: APIFY_PERSIST_STORAGE=0 → APIFY_PURGE_ON_START=0.
• Actor lifecycle: fixed an empty <ApiLink>.
• Quick-start: corrected the __main__.py description and tab label.

Consistency & additions

• Consistent capitalization ("Apify Proxy", "Apify Console", "Actor"); assorted grammar and upgrade-guide heading fixes; made an absolute self-link relative.
• New Storage clients concept page with two runnable snippets (default SmartApifyStorageClient, single vs. shared request queue, force_cloud, custom client via service_locator); re-exported StorageClient from apify.storage_clients.
• Added a ## Conclusion to every concept page, expanded the Introduction feature list, and added a Configuration options table.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.98%. Comparing base (3f25d4a) to head (7ca6d59).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
+ Coverage   86.87%   86.98%   +0.10%     
==========================================
  Files          48       48              
  Lines        2942     2942              
==========================================
+ Hits         2556     2559       +3     
+ Misses        386      383       -3     

Flag	Coverage Δ
e2e	37.76% <100.00%> (ø)
integration	59.14% <100.00%> (+0.03%)	⬆️
unit	75.69% <100.00%> (+0.06%)	⬆️

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