Documentation for tools to work with odoc

[dlesbre]

The odoc doc is one of the first references I found when trying to write documentation for OCaml, its has good referencing and is easy to read. I believe it could be improved by presenting (or at least mentioning) the following:

• having sherlodoc installed generates a search bar, this is a nice feature and extremely easy to use, but AFAIK it isn't mentioned anywhere in the docs.
• mdx can be used to run odoc code blocks and check their output, thus ensuring you can't forget to update the documentation examples when updating the code.
• the @doc-json target is useful to customize the output of odoc and integrate it in one's own website, it could do with more publicity both on the dune side (Better documentation for @doc-json dune#10557) and in the odoc docs.