fix(docs): give the introduction page a descriptive title, description and H1

[BryanFRD]

Closes #1314

• docs-en/introduction.md: title What is FerrFlow? Semantic versioning CLI (rendered as ... : FerrFlow Docs), a description that says what the binary does, and an # What is FerrFlow? heading.
• docs-fr/introduction.md: the same in French.
• The page was at position 5.3 with a 0.36% CTR over 279 impressions: well placed and rarely clicked, so the snippet is the lever.
• The nav label stays Introduction (it comes from DOCS_NAV, not the frontmatter).
• Frozen snapshots (docs-vN) are untouched.

[ferrfleet]

Nothing blocking. Frontmatter is valid, the EN and FR pages match, and the frozen snapshots are untouched.

Nit: the FR strings are long for a search result. With the : FerrFlow Docs suffix the title is about 70 characters and the description about 190, so Google will probably truncate both. If French snippets matter, shorten them, for example Qu'est-ce que FerrFlow ? Versionnage sémantique.

Nit: no other page in docs-en has a Markdown H1, so this one is now the odd one out. I couldn't check the renderer from this repo, so it's worth confirming that the frontmatter title isn't also rendered as an H1, which would give the page two.