docs: convert filesystems WG proposal into a charter

Follow the Group Charter Template and the WG formation flow: move the
document to docs/community/working-groups/filesystems.mdx and add the
navigation entry to docs/docs.json.

Facilitator(s) becomes a Leadership table listing WG Leads. Adds the
Group Type, Out of Scope, Related Groups, Authority & Decision Rights,
Membership, Operations, and Changelog sections.

Correct the referenced proposals against GitHub. SEP-1708 is closed,
SEP-2571 and SEP-2532 are pull requests, and all three are now cited by
their full titles. Drop the unverified method count for the files/*
family.