Improve discoverability, SEO, and cross-linking across docs

[mattsodomsky]

Summary

• Frontmatter descriptions added to ~20 section READMEs and reference pages that were missing meta descriptions (task code, options, previews, permissions, Liquid basics, filters, tags, objects, GraphQL, integrations, email, events, and more)
• Deprecation dates fixed across 34 REST API object pages — changed future-tense "will cease to work on Feb 1, 2025" to past tense, since that date has passed. Also fixed a typo ("competely") in blog.md
• Cross-links added to key dead-end pages: HTTP action → external APIs/responding to results, Email action → platform email/templates, Shopify action → reading/writing data, Cache → debouncing/batch processing, Task code → Liquid reference pages, Preventing action loops → related techniques
• Execution flow section added to core Events page explaining Event → Task Run → Action Run
• Nav item added for event topics list under Core Concepts > Events

Test plan

• Verify GitBook renders frontmatter descriptions correctly
• Spot-check cross-links resolve to correct pages
• Confirm deprecation language reads naturally in past tense
• Check SUMMARY.md event topics list appears in nav