docs: fix link to page-constructor sandbox

[martyanovandrey]

The sandbox link on the Page constructor page pointed to the Editor/Main story in the main page-constructor Storybook. That story disappeared from the deployed Storybook after gravity-ui/page-constructor#1426 renamed Editor.stories.tsx to Editor.tsx (the file no longer matches the stories glob), so the link opens a "story not found" page.

Point to the Storybook preview of gravity-ui/page-constructor#1116 instead, where the same Editor/Main story is still built and works. Editor 2.0 in that preview is not usable yet (its preview iframe is hardcoded to localhost:3000), so the old editor is the only working sandbox for now.

Changed in both ru and en versions of project/page-constructor.md.

Superseded by #342, which points the link to the Diplodoc playground instead of a per-PR preview.

[github-actions]

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⚠️ en/guides/openapi.md: 168: YFM001 / inline-code-length Inline code length [Expected: 100; Actual: 265] [Context: " — the specification is displayed directly on the page (default value). If the size of the specification's json schema exceeds the value of ##maxOpenapiIncludeSize## (100 KB by default), the mode automatically switches to "]

⚠️ en/tools/docs/build.md: 50: YFM001 / inline-code-length Inline code length [Expected: 100; Actual: 109] [Context: " — a snapshot of the dependency graph (pages, included files, assets, missing paths, edge count), as well as "]