Adding reference pages for HandlerBuilder

[egilmorez]

Adding this namespace per an internal bug/feature request.

Staged internally at https://firebase.devsite.corp.google.com/docs/reference/functions/handler_builder_.handlerbuilder?skip_cache=true

[laurenzlong]

The doc changes to the other files look really good. Unfortunately due to the way that we implemented src/handler-builder.ts the auto generated references are not particularly usable at the moment, so we'll have to rethink the structure. We can still get this PR merged once you've addressed my comments, because it is a good start, but let's sync on how we actually document the handler namespace better. Also can you run npm run format and commit the changes? That's what's causing the build to error out.
@rachelsaunders Don't review the function-specific docstrings in handler-builder.ts since they'll change dramatically but you can review the "disclaimer text" at the top.

[laurenzlong]

the title should be functions.handler, since that's how a developer actually writes it in code.
e.g.
exports.myFunction = functions.handler.database.ref.onWrite(...)

[laurenzlong]

This section can also be lower in the list (it can be the last item actually, since regular CF3 developers won't need it, and extensions docs will link directly to the handlers page.)

[laurenzlong]

Revise to:

"The HandlerBuilder class facilitates the writing of functions by producers of Firebase Extensions and developers that wish to use gcloud CLI or Cloud Console to deploy their functions. "

(I would actually remove the part of limited beta, it is not true, it's actually in EAP, and we also will definitely forget to update this comment later)