docs: point page-constructor sandbox link to Diplodoc playground

[martyanovandrey]

Follow-up to #341. That PR pointed the sandbox link at the Storybook preview of gravity-ui/page-constructor#1116, which is a per-PR artifact and will go away once that PR is merged or closed.

Point to the Diplodoc playground in Page constructor mode instead: https://diplodoc-platform.github.io/playground/?prefill=true&mode=PC. With prefill=true the playground loads its built-in sample page, so a long input= query is not needed (a URL with the full config inlined gets 414 from GitHub Pages).

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

[github-actions]

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Warnings (2)

⚠️ 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 "]