docs: Fix errors and standardize frontmatter across all pages

[vdusek]

Summary

• Fix error handling examples — except Exception as ApifyApiError was catching all exceptions and shadowing the class name; now properly uses from apify_client.errors import ApifyApiError with except ApifyApiError as err
• Fix retries page — default retry count corrected from 8 to 4 (matches DEFAULT_MAX_RETRIES); updated outdated min_delay_between_retries_millis to min_delay_between_retries (takes timedelta)
• Fix convenience methods page — start() was described as waiting for finish (it doesn't); added missing wait_for_finish() method
• Fix nested client examples — replaced ty: ignore comments with proper ActorJobStatus.SUCCEEDED enum usage
• Fix README — updated Quick Start to typed attribute access (actor_call.default_dataset_id), corrected retry count and parameter name
• Fix comment grammar — "waits" → "wait" in code examples
• Add description frontmatter to all 20 doc pages for SEO
• Standardize guide titles to imperative form ("Pass input to an Actor", "Integrate with data libraries", "Use HTTPX as the HTTP client")

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (2f67a5d) to head (8e6ad44).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #695      +/-   ##
==========================================
+ Coverage   94.79%   94.84%   +0.04%     
==========================================
  Files          45       45              
  Lines        4403     4403              
==========================================
+ Hits         4174     4176       +2     
+ Misses        229      227       -2     

Flag	Coverage Δ
integration	94.84% <ø> (+0.04%)	⬆️

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