Extract and document potentially sensitive links to ensure safely making the repo public

[griff-rees]

• The link in FULLTEXT_SAS_TOKEN="?sv.." in docs (README.md and fulltext.md) might be problematic when making this repository public. #130
• The .env/local example config is meant to be only used privately, never for public hosting, but ensuring that's documented well to avoid risks (and extract anything extra from that) #129

[griff-rees]

Also see #128

[griff-rees]

The remaining task here is documentation. As best I can glean, @AoifeHughes and I have sorted the purging needed, just thought it best to document that to help prevent it happening again. That can be treated as another ticket with less urgency if easiest to close this first.