docs: reorganize cookbook

[frydj]

Summary

Moves cookbook into directory structure:

• index.md: Cookbook overview and categories
• examples.md: (moved from cookbook.md) Recipe collection

Test plan

• Verify examples are still accurate
• Check internal links resolve correctly

[frydj]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs: add feature showcase examples #71
• docs: add domain example datasets #70
• docs: add feature reference docs #69
• docs: reorganize cookbook #68 👈 (View in Graphite)
• docs: restructure querying reference #67
• docs: restructure HTTP API reference #66
• docs: add advanced topics #65
• docs: add security guides #64
• docs: add modeling guides #63
• docs: add working with data guides #62
• docs: reorganize learn foundations #61
• docs: add getting started guides #60
• docs: add project config & infrastructure #59
• main

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