fix: dedupe heading slugs to prevent duplicate ids on doc pages

[sanketssc]

Fixes #1281

When a module exposes a class and a type with the same name (e.g. the Spring class and the legacy interface Spring<T> in svelte/motion), the generator emits two ## Spring H2s. The renderer assigned both the same id="Spring", which:

• produced invalid HTML (duplicate ids on the page),
• broke in-page anchors (/docs/svelte/svelte-motion#Spring only ever reached the first heading, the legacy interface section was unreachable),
• broke the "On this page" TOC (clicking either Spring link highlighted both at once).

This PR dedupes heading slugs per page. The first occurrence keeps the bare slug, subsequent occurrences get -1, -2, … suffixes. The same dedupe is applied in two places so they stay in sync:

• packages/site-kit/src/lib/markdown/renderer.ts — heading id + permalink href in rendered HTML.
• packages/site-kit/src/lib/server/content/index.ts — sections/subsections metadata that powers OnThisPage.svelte.

Result on /docs/svelte/svelte-motion: first Spring (class) stays at #Spring, legacy interface section becomes #Spring-1. Each TOC entry now activates independently.

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Pull request overview

This PR addresses duplicate heading id values on docs pages by de-duplicating generated heading slugs per page, keeping rendered heading anchors and the “On this page” TOC metadata aligned.

Changes:

• Add per-page slug de-duping when generating sections/subsections metadata for the docs TOC.
• Add per-page slug de-duping when rendering heading id attributes and permalink hrefs in markdown output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/site-kit/src/lib/server/content/index.ts	De-dupes sections and subsections slugs so TOC entries remain unique per page.
packages/site-kit/src/lib/markdown/renderer.ts	De-dupes rendered heading ids / permalinks to avoid duplicate IDs in generated HTML.