docs: add Bitsocial case study

[Rinse12]

What

Adds a case study for Bitsocial, an open-source protocol for decentralized social media that uses IPFS and libp2p as its entire backend (no central servers, no blockchain, no federation).

The page follows the structure of the existing Vereign case study: Overview, What is Bitsocial, The story, How Bitsocial works, How Bitsocial uses IPFS, IPFS benefits, Operational lessons, and a forward-looking section.

Highlights of the IPFS/libp2p usage covered

• Public-key addressing via IPNS — a community is named by the hash of its public key and publishes signed IPNS records pointing to its latest content CID.
• Content addressing for posts and paginated community updates.
• libp2p gossipsub for the publish/anti-spam-challenge handshake.
• Helia in the browser, Kubo on community/self-host nodes.
• Delegated HTTP routing for fast IPNS resolution and provider discovery.
• An Operational lessons section with real-world findings (browser-dialable provider transports, provider-record expiry/reprovide lag, isolating per-router failures in discovery).

Changes

• docs/case-studies/bitsocial.md — new case study
• docs/.vuepress/config.js — registers the page in the navbar and both sidebar lists
• .github/styles/pln-ignore.txt — adds bitsocial and seedit for the Vale spellcheck

Notes for reviewers

• No adoption metrics were invented; the project's public materials don't publish figures.
• Project reference: https://bitsocial.net/ and https://bitsocial.net/docs