ci: add Lychee link checker for the docs

[vdusek]

Adds a scheduled link checker for the docs, mirroring crawlee-python's scheduled-tests workflow. Three parts:

• .github/workflows/on_schedule_lychee.yaml: runs on weekdays at 01:00 UTC (and on manual workflow_dispatch), builds the docs with uv run poe build-docs, and runs lycheeverse/lychee-action over the generated ./website/build/**/*.html to catch broken external links and runtime 404s. On a failed scheduled run it posts a Slack alert to the team channel via SLACK_WEBHOOK_URL (skipped on manual dispatch, so ad-hoc triggers don't spam). It is not PR-blocking.
• .lycheeignore: ignore patterns ported from apify-docs and retargeted to this repo.
• onBrokenLinks / onBrokenMarkdownLinks → throw in docusaurus.config.js: broken internal links now fail the docs build (anchors stay at the default warn, same as apify-docs).

The broken-link fixes this throw policy depends on already landed in #997. Verified locally: full poe build-docs with throw passes (0 broken links).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.51%. Comparing base (726620b) to head (8a27173).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #998      +/-   ##
==========================================
- Coverage   90.54%   90.51%   -0.04%     
==========================================
  Files          49       49              
  Lines        3132     3132              
==========================================
- Hits         2836     2835       -1     
- Misses        296      297       +1     

Flag	Coverage Δ
e2e	36.14% <ø> (ø)
integration	57.12% <ø> (-0.04%)	⬇️
unit	79.27% <ø> (ø)

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