Update NESO hyperlink now showing status code 403

[sadielbartholomew]

The National Grid ESO website now lives under a new domain so the URL we use goes to an intermediate page with a 403 stats code:

$ curl -i https://www.nationalgrideso.com/
HTTP/2 403 
date: Thu, 09 Apr 2026 16:19:17 GMT
content-type: text/html
cache-control: private, max-age=0, no-store, no-cache, must-revalidate, post-check=0, pre-check=0
expires: Thu, 01 Jan 1970 00:00:01 GMT
referrer-policy: same-origin
...

showing:

so this PR updates the link, where used, the new location. From a git grep grideso we only use the old link in one place, so only one instance needs updating - in the docs 'Introduction' page.

[abhidg]

Looks good, thanks @sadiebartholomew