Add digital.nhs.uk links to .lycheeignore

These have started consistently failing with 403 errors in the links
checks. All are accessible in a browser.

.lycheeignore

@@ -27,6 +27,10 @@ https?://twitter\.com(?:/.*$)?$
 # An example OpenSAFELY project that is not intended to exist.
 https?://github\.com/opensafely/example-research-repository(?:/.*)?$
 
+# digital.nhs.uk links used to work but are now failing with a 403
+# Network error: Forbidden
+https?://digital.nhs.uk(?:/.*$)?$
+
 # The following ignore line ignores this link in the source: ../data-sources
 #
 # MkDocs actually uses an index.md file to generate this,