docs: rewrite GitHub integration page as step-by-step tutorial

[marcel-rbro]

Closes #2492

Summary

Rewrites sources/platform/integrations/programming/github.md:

• Restructured as Prerequisites → Create an Actor from a GitHub repository (Steps 1-3) → Build automatically on every push → Create a GitHub issue when an Actor run fails (Steps 1-4) → Resources.
• Updated the new-Actor walkthrough to match the current Apify Console UI ("Let's build a new Actor!" page, with GitHub listed directly under Link a Git repository - no intermediate provider modal).
• Replaced the bare YouTube embed for the failure-issue flow with a written tutorial: generate a fine-grained GitHub PAT, add an ACTOR.RUN.FAILED HTTP webhook on the Actor, set the headers/payload templates to call POST /repos/{owner}/{repo}/issues, save and test. Kept the original video as a "Video walkthrough" subsection.
• Refreshed two screenshots: apify-git-repository.png (the new-Actor page) and integrations-tab.png (the Add integration page; this image is also reused by slack.md and webhooks/index.md).
• Removed the now-unused apify-git-provider-select.png (the provider-picker modal no longer exists).
• Adopted the current intro convention (plain prose + <ThirdPartyDisclaimer />, no bold one-liner or --- rule), matching recent integration pages (Pipedream, Snowflake, Manus, viaSocket, x402).

Reviewer question

There's an inline MDX comment in the Build automatically on every push section asking whether the native "Link a Git repository" flow already wires up auto-builds (the panel's helper text suggests it does). If so, the manual GitHub webhook recipe currently in that section is the legacy path - please confirm and let me know whether to keep it as-is, replace it with a one-line note, or keep both with a clarifying intro.

Test plan

• pnpm lint:md passes for the changed file
• vale "sources/platform/integrations/programming/github.md" reports 0 errors
• All cross-link slugs verified to exist (/platform/actors/development/deployment/continuous-integration, /platform/actors/development/deployment/source-types#private-repositories, /platform/integrations/webhooks/events, /platform/integrations/webhooks/actions)
• pnpm build passes (no broken links, frontmatter valid)
• Page renders correctly in dev server: prereqs, all numbered steps, admonitions, code blocks, and the embedded YouTube iframe
• Slack and Webhooks pages still render the (replaced) integrations-tab.png correctly

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