Missing parameter injection documentation

[src/site/resources/reference.html](https://github.com/stapler/stapler/blob/master/src/site/resources/reference.html#L99) has a link to `parameter-injection.html` but this document does not exist. Please document parameter injection, it seems to be an important part for Jenkins plugins.

Start at src/site/resources/reference.html around line 99 and verify how it links to parameter-injection.html. Read the existing Stapler documentation and parameter-injection behavior to determine the concepts and examples that must be covered. Done means adding the missing parameter-injection.html document and making the reference link resolve correctly.

Written by the indexing model from the issue text.