Review all front-end-related documentation

[GuySartorelli]

Our front-end documentation is fractured, outdated, and in some cases potentially misleading. It could do with a review.

Some things to consider:

• https://docs.silverstripe.org/en/4/developer_guides/customising_the_admin_interface/reactjs_redux_and_graphql/#introduction-to-the-react-layer says the react API isn't part of our semver commitment, but that seems to be the only place that mentions it. It certainly isn't mentioned in our major release policy, which is the place where most people will look for that sort of information.
• It would be nice if there was a clearer structure to the doc - currently it seems to find information about javascript development in the CMS we have to traverse all over the place. These should be collected in a single section, ideally (see Restructure "customise the CMS" docs #41)
  • In particular, the react injector documentation is very difficult to parse. It needs to be split into multiple pages. At the very least, there should be one page that focuses on the API of the react injector, and another that provides concrete step-by-step examples of how to use it in a specific narrow-scope project. At the moment those are mixed together, which ultimately means the step-by-step guide is hard to follow at best, and the documentation about the API isn't clear nor complete. See DOC Update react injector docs to work #215 (comment)
• A lot of our graphql js stuff is outdated (see Update docs from graphql 3 to graphql 4 compatability #62)