[Cashtab] Adding stub dev docs with facebook docusaurus

Summary: This addition was created with the command npx @docusaurus/init@latest init per https://v2.docusaurus.io/docs/installation. It generates stub doc pages. This diff is intended to serve as a reference point for future customizations to developer docs for Cashtab extension, components, and bch-api.

Test Plan:

cd web/docs
npm i
npm start

Navigate to localhost:3000 if it does not open automatically in browser. Observe default docusaurus stubs.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9087