feat(components): add SIRET reminder banner to FR account guides (#311)

Add a reusable, manually-triggered <Banner> component (the new-docs
successor to the legacy templates/banners HTML fragments) and place a
SIRET-completion banner on the FR account-information guides.

The banner is FR-only (SIRET is a French company-registration concept),
follows the Campaign Creative Guidelines longtext format, links "numero
SIRET" to the FAQ #siret anchor, and CTAs to the Control Panel profile.

- components/Banner: component, styles, doc-check icon; registered as a
  global MDX component in both rspress configs
- faq-account-management: add id="siret" anchor on the SIRET Q&A
- 23 account-information FR guides: insert the banner tag