docs(website): add the comparison page

[dangreen]

Draft. Stacked on #234: the base is chore/effect-benchmark-refresh, so this diff shows only the comparison page; GitHub retargets it to main once that PR merges.

What

New getting-started/comparison page, second item in Getting Started, with the Introduction pinned first in the sidebar.

• Store layer: Nano Stores, Zustand, Jotai. Feature table, size table, then Nano Kit vs … for each.
• Query layer: @nanostores/query, TanStack Query. Same structure.
• Router layer: @nanostores/router, TanStack Router. Same structure, plus three @nano_kit/router import sets so the TanStack row compares like for like.
• Trade-offs: what Nano Kit gives up today and what it gives back for it.

Numbers

Sizes: Vite 8.2.1 with rolldown 1.2.7, app build of one entry that imports the listed names, default minifier, ES2022, gzip level 9, versions listed in the note at the top of the page. Benchmark figures quoted in the text come from the tables refreshed in the base PR.

Every claim about another library was checked against that library's README or exports; every size claim is tied to the measured import set.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.79%. Comparing base (52a147b) to head (c510505).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           docs/changes-since-1.2.0     #235   +/-   ##
=========================================================
  Coverage                     85.79%   85.79%           
=========================================================
  Files                           166      166           
  Lines                          3766     3766           
  Branches                        871      871           
=========================================================
  Hits                           3231     3231           
  Misses                          358      358           
  Partials                        177      177           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.