Link the PluralPort Converter from the header and docs

[SiteRelEnby]

Summary

The PluralPort Converter turns an export from an app that does not write PluralPort files yet into one, in the browser, and its source mappings double as reference implementations of the spec. The site did not mention it anywhere. This adds it in the places a reader would look for it.

• Header and footer: a Converter entry, on desktop and in the mobile menu, marked as an external link.
• Homepage: a third hero link ("Convert an export") and a paragraph under "What the problem looks like" explaining what the converter is for.
• Adopt: a "Want a worked example?" callout pointing at the converter and its source mappings, since they do steps 1 to 3 of the quickstart for real apps.
• About: the licensing card now lists the converter as MIT alongside the validator and schemas.
• Format index: a one-line pointer for readers who want to move their data rather than implement the format.
• App pages: the four sources the converter reads today (Ampersand, BerryTree, PluralKit, Octocon) get a "Convert an export to PluralPort" link. BerryTree's reader implementations section also lists what the converter covers.

The converter has no per-source deep link, so all app-page links go to its root.

Verification

Static build passes. Homepage, Adopt, and the BerryTree page were rendered from the build and checked visually, no console errors.