docs: document Slack integration custom message format

[marcel-rbro]

Summary

Closes #2490 by documenting the Slack integration's custom message template format and the full list of template variables. Also tightens existing prose on the page.

New reference content

• ## Custom message format — Handlebars template engine, Slack mrkdwn syntax, 2–4000 char length limit, auto-appended Sent by Apify line, TEST event behavior, and empty-template fallback.
• ## Template variables with sub-sections:
  • ### Event data — actorId, actorTaskId, actorRunId, actorBuildId
  • ### Resource (Actor run) — full table of frequently used resource.* fields (id, status, statusMessage, startedAt, finishedAt, defaultDatasetId, options., stats., meta.origin, etc.)
  • ### Resource (Actor build) — build-specific resource fields
  • ### Status values — split list for runs vs builds
  • :::tip Full resource object admonition pointing to the Get run / Get build API responses
• ## Message template examples — four ready-to-paste templates: minimal status notification, run summary with Apify Console links, failure alert with {{#if}} conditionals, and build notification.
• Cross-link from Step 1 to the new Custom message format section so the existing setup flow points readers at the new reference.

Page cleanup (existing content)

• Intro tightened — removed the lead paragraph that duplicated the frontmatter description verbatim, collapsed two separate tutorial pointers (one was a blockquote with an orphan --- rule above it) into one, and replaced the wordy value paragraph with a single value-forward sentence.
• Image alt text — replaced generic alt text on all three screenshots (Integrations tab, Integration setup, Use Apify from Slack) with descriptive text.
• Step 1 prose — switched to active voice ("Log in ... when prompted"), dropped wordy phrasings ("events you would like to be notified of", "Once you are done"), and restructured the long event-selection sentence.
• Misc style — removed redundant "the" before [Google Shopping Scraper], replaced "feel free to reach out to us on our [Discord channel]" with "reach out on the [Apify Discord channel]".

Variable accuracy

Variables and their structure are grounded in the integration's source code (slack_webhook_handler.ts and webhooks.ts schema). Status values match what's exposed by the integration handler.

Test plan

• pnpm lint:md passes
• vale --minAlertLevel=error passes
• Front matter description within 140-160 chars (147)
• On the deploy preview, verify the #status-values anchor link works and the Apify Console URL patterns in the example templates resolve

Closes #2490

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