Link to codebooks more prominently

[Robinlovelace]

I can only see the codebook links when inside the app and clicking on 'Region Data'. I think they could also usefully linked to here: https://www.pct.bike/tabs/about.html

I would like to broaden this issue to be 'add new links to about tab': we can add links to the essentials document and other content now also I think. But will await to see if this ideas gets 👍 from others, thoughts @JDWoodcock, @RachelAldred, @AnnaGoodman1, @usr110 and @nikolai-b ? Sorry for mass pinging but thing this could be low hanging fruit fixed in a few lines of (html) code.

[AnnaGoodman1]

linking to essentials doc from 'about' tab sounds fine to me. I don't think code books are very self-explanatory, only make any sensei with the data, so think they should just be linked to from the one place.