Fix: Tesla Powerwall setup instructions and example

[mattsday]

The examples given for Tesla setup did not quite work in my setup when done today (2026-05-28).

Specifically:

1. The mechanism to generate an access token is currently not working due to a reported issue - this fix offers an alternative mechanism either through the official documentation (somewhat complex) or through a SaaS service (ymmv on risk appetite) (+new commit instructs on using Fleet API integration to obtain an access token much more simply)
2. The access token generated today was longer than 1020 characters so a 5th input field needed to be added. This PR adds that.
3. New: Changes the API to use fleet-api.prd.eu.vn.cloud.tesla.com identified by @allanPHD in owner-api.teslamotors.com dead for energy sites (May 2026) — Fleet API migration fix #3965