docs: remove broken links to app.deepchecks.com

[YaronFriedman]

Summary

• The hosted app at app.deepchecks.com is no longer available, so any link/reference to it in the docs is broken.
• Strips the URL from the top-nav, prose, RST hyperlinks, and code-comment examples across the user guide and getting-started pages.
• Replaces the URL inside the alert-webhook JSON sample with a neutral your-deepchecks-host placeholder so the example still reads.
• Surrounding tutorial flows are left untouched per scope — only the dead link itself was removed.

Test plan

• make docs (or equivalent Sphinx build) succeeds without warnings about removed targets.
• Visually scan the rendered nav, the affected pages, and the sphinx-gallery examples to confirm the prose still reads cleanly.
• grep -r "app.deepchecks.com" docs/ returns no hits.

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