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

[vdusek]

Reviews the SDK documentation against the codebase, fixes verified inaccuracies, unifies wording and structure, and fills several gaps.

Correctness fixes (verified against the code)

• Storages: Dataset.export_to_csv/export_to_json don't exist → documented the real Dataset.export_to method (content_type='csv'|'json').
• Upgrading to v2: Actor.start_task never existed → Actor.call_task (accepts Webhook, returns ActorRun).
• Introduction: Python requirement 3.10+ → 3.11+ (matches requires-python = ">=3.11").
• Web server guide: Actor.configuration.container_url/container_port don't exist → web_server_url/web_server_port (the prose contradicted the snippet, which was already correct).
• Actor events: corrected the SYSTEM_INFO payload (handlers receive EventSystemInfoData with cpu_info/memory_info), MIGRATING data (time_remaining), and Config → Configuration; each event now links its real data type.
• Configuration: replaced the stale APIFY_PERSIST_STORAGE=0 example with APIFY_PURGE_ON_START=0.
• Actor lifecycle: fixed an empty <ApiLink to="">.
• Events snippet (04_actor_events.py): fixed a latent runtime bug — passing a Pydantic model as logging extra= raises TypeError; now uses event_data.is_migrating.
• Quick-start: corrected the __main__.py description (the shown snippet doesn't configure logging).

Consistency & wording

• Consistent capitalization of "Apify Proxy", "Apify Console", and "Actor" (headings + prose).
• Fixed __main.py__ → __main__.py tab labels/titles (quick-start, Scrapy guide).
• Made an absolute self-link relative; fixed upgrade-guide heading wording and hierarchy (v3, v4).
• Assorted grammar (force_cloud, "a URL", "Twisted's", missing spaces, …).

Additions

• New Storage clients concept page (12_storage_clients.mdx) + two runnable snippets: default SmartApifyStorageClient, single vs. shared request queue, force_cloud, and custom-client setup via service_locator. Cross-linked from the storages page and quick-start.
• Re-exported StorageClient from apify.storage_clients.
• Added a ## Conclusion section to every concept page for consistent structure.
• Actor Standby section in the web server guide.
• Expanded the Introduction feature list and added a commonly-used-options table to the Configuration page.

Verification

ruff check docs/ and ty check pass; the new/edited snippets run to exit code 0. The Dataset.export_to and Actor.call_task corrections were confirmed by runtime introspection and git history, and the ActorLogFormatter ApiLink was verified to resolve (a fresh API-ref regen plus a live HTTP 200), so it was kept as-is.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.87%. Comparing base (0f4dd21) to head (b815385).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
- Coverage   86.94%   86.87%   -0.07%     
==========================================
  Files          48       48              
  Lines        2942     2942              
==========================================
- Hits         2558     2556       -2     
- Misses        384      386       +2     

Flag	Coverage Δ
integration	58.90% <100.00%> (ø)
unit	75.62% <100.00%> (-0.07%)	⬇️

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