[docs-cashtab] night mode default and twitter preview card

Summary: Modify themeConfig settings to set dark mode to default and add a twitter preview banner.

Test Plan: npm start and observe loads in dark mode. Twitter preview card will need to be tested after deployment.

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D9203