New links to key resources on website

[Robinlovelace]

• Link to training resources in Manual, e.g.: Add link to training materials #831
• Link to essentials document from About page
• Link to codebooks from About page Link to codebooks more prominently #832
• Link to codebooks from Manual Link to codebooks more prominently #832

We don't need to add all of them but though it worth documenting, I've tried to put the links in descending order of importance, there may be more links worth adding.