Editorial: update to modern Web IDL conventions (#1528)

* Editorial: update to modern Web IDL conventions

* Uses "getter steps" and "method steps"
* Use "this" instead of "context object"
* Talk about dictionaries as Infra maps
* Minor restructuring of promise-creation/in parallel algorithms for NavigationPreloadManager

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

* Update docs/index.bs

Co-authored-by: Jake Archibald <[email protected]>

Author: domenic