feat(seo): add titles and meta descriptions to overview pages (#82)

* feat(seo): add titles and meta descriptions to overview pages

Rspress core's Layout only honours `frontmatter.title` for `doc`/`doc-wide`/
`home`/`404` pageTypes. Custom pageTypes (`overview`, `migration`, `elearning`)
fell through to the bare site title, so every product overview shipped as
`<title>OVHcloud Documentation</title>` with no meta description.

Theme fix
- New `theme/hooks/usePageTitle.ts` calls `useHead` after Rspress core's own
  HeadTags so unhead's last-write-wins keeps the page-specific title.
- Wired into OverviewLayout, MigrationLayout, and ELearningLayout.

Content fix
- Added a `description:` frontmatter field to every overview page across all
  7 locales (58 EN/DE/ES/IT/PL/PT + 61 FR including 3 FR-only overviews:
  internet-access, overthebox, fax).
- Descriptions authored in EN, then translated to the 6 other locales using
  the project terminology CSV for consistent product naming.

* fix(seo): replace em dashes with hyphens in overview meta descriptions

SEO team feedback: em dashes (—) can trip up snippet renderers and copy-paste
flows; plain ASCII hyphens are safer. Scoped strictly to top-level
description: lines so CTA descriptions and other content are untouched.

Author: Y0Coss