fix(docs): build-time plugin count on the docs index

Part of the Markdown for Agents Phase 1 epic (see #5652). kestra.io has Cloudflare Markdown for Agents enabled: `Accept: text/markdown` converts the SSR HTML at the edge, so defects in the HTML end up in what AI agents read. Cloudflare strips `nav`/`header`/`footer`/scripts/styles/inline SVG, ignores `aria-hidden` and empty `alt`, keeps `.visually-hidden` text and `display:none` content, and renders `

` as separate lines (all verified on production).

## Problem
`src/contents/docs/index.mdx` uses `` (`src/components/content/PluginCount.vue`), whose `usePluginsCount()` starts at `0+` and fetches on the client. SSR ships `0+`, so `/docs` reads "Thanks to 0+ plugins" in HTML and in the converted Markdown. Not covered by #5446, which fixed `/enterprise` and `/1-0`.

## Scope
- Add `src/components/content/PluginCount.astro` that awaits `fetchTotalPluginsCount()` and renders the formatted value; use it from `index.mdx` (Astro components work in MDX).
- Leave `PluginCount.vue` for callers that pass `subgroups`, or remove it if unused after the switch.

## Done when
- `dist/client/docs/index.html` contains the real count; no `0+` in `/docs` Markdown.

Read src/contents/docs/index.mdx and src/components/content/PluginCount.vue, then trace fetchTotalPluginsCount() before adding the Astro component named in the issue. Build the docs and inspect dist/client/docs/index.html; done means it contains the real plugin count and the /docs Markdown no longer contains 0+.

Written by the indexing model from the issue text.