testsite: README update

[1amcode]

Updating commands in testsite/README.md as the current ones do not work.

As below, from testsite/:

$ yarn start
Usage Error: Couldn't find a script named "start".
> yarn run [--inspect] [--inspect-brk] [-T,--top-level] [-B,--binaries-only] <scriptName> ...

$ yarn testsite:start
[INFO] Starting the development server...
[SUCCESS] Docusaurus website is running at http://localhost:3000/.

✔ Client
  Compiled successfully in 2.87s

client (webpack 5.72.0) compiled successfully

The same for build:

$ yarn build
Usage Error: Couldn't find a script named "build".
> yarn run [--inspect] [--inspect-brk] [-T,--top-level] [-B,--binaries-only] <scriptName> ...

$yarn testsite:build
[INFO] [en] Creating an optimized production build...

✔ Client


✔ Server
  Compiled successfully in 19.96s


✔ Client


● Server █████████████████████████ cache (99%) shutdown IdleFileCachePlugin
 stored

[SUCCESS] Generated static files in build.
[INFO] Use `npm run serve` command to test your build locally.

(FWIW yarn install is ok)

[RDIL]

LGTM, thanks!