[cashtab-docs] patch title so twitter card shows distinct title, tagline

Summary: Page title currently reads "Build on BCHA | Build on BCHA" due to Docusaurus default settings. Correct so it instead reads Build on BCHA | Cashtab Docs"

Test Plan: 'npm start' and observe new title

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9205