Docs: translate site content to Français (fr)

[pathosDev]

The docs site at https://actor-ts.dev ships UI infrastructure for fr since commit 691d6b0 — the language switcher, sidebar, and Starlight UI chrome all render in Français. Page content currently falls back to English.

This issue tracks the full content translation of docs/src/content/docs/**/*.mdx into Français.

Scope

• Translate ~900 pages from the canonical English source under docs/src/content/docs/ (top-level *.mdx, excluding the de/ and api/ subdirectories). Always work from the EN files — not from another locale's translation — so that all locales stay consistent and don't compound drift from intermediate translations.
• Preserve the technical glossary unchanged across locales (Actor, Mailbox, Supervisor, CRDT types, protocol names, runtime names). The same conventions used for the DE rollout apply.
• Cross-section internal links rewrite their prefix to /fr/....
• The migration pages (migration/from-akka-jvm.mdx, from-pekko.mdx, from-akka-net.mdx, from-orleans.mdx, from-vanilla-ts.mdx) keep technical comparisons verbatim but translate the narrative prose.

Out of scope

• Auto-generated API reference under docs/src/content/docs/api/ (regenerated from JSDoc on every CI run; stays English).
• Sidebar labels (already translated as part of the infra commit 691d6b0).

How to contribute

• Pick a section (e.g. intro/, fundamentals/, cluster/).
• Open a draft PR with the translated MDX files under docs/src/content/docs/fr/<section>/, translating directly from the EN source files in docs/src/content/docs/<section>/.
• If a stylistic question comes up about the actor-ts terminology, the existing DE translation under docs/src/content/docs/de/<section>/ can be consulted as a precedent — but it is not the source. Source is always EN.

Labels: documentation, i18n.