[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