Add APM sizing calculator docs page

[anirudha]

Summary

• New documentation page under Application Monitoring: Sizing Calculator
• Documents all calculator inputs, formulas (span storage, service map, RED metrics), key assumptions, and capacity planning tips
• Links from the APM overview page under a new "Sizing and capacity planning" section

Test plan

• starlight-links-validator passes — all internal links valid
• Local preview at http://localhost:4321/docs/apm/sizing-calculator/ returns 200
• Visual review of page layout and table formatting

Signed-off-by: Anirudha Jadhav anirudha@nyu.edu

[github-actions]

✅ Docs preview build completed successfully! All tests passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.96%. Comparing base (e53054c) to head (15632c6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
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  Coverage   54.96%   54.96%           
=======================================
  Files           4        4           
  Lines         151      151           
  Branches       42       43    +1     
=======================================
  Hits           83       83           
  Misses         66       66           
  Partials        2        2           

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