Add a sphinx plugin to allow linking from demo configurations to api sections

### description

learning envoy configuration can be quite a steep learning curve and its not always easy to navigate the docs to find the relevant spec

im thinking it might be helpful if the config examples in the docs provided a click through to the relevant api section - perhaps if you hover over the line a link appears at the end, for example

depending on how fine grained the links were, i think we might need to add more ref links in the api to allow that

on a related point im noticing that ~all~ most of the api examples are `json` - whereas the config examples are mostly `yaml`. There are probably good reasons for both, but im thinking this is another way in which the docs can jar for new users.