documented better developer experience

[softprops]

Issue #, if available:

Description of changes:

It's been noted in a few open issues that the developer experience for getting a basic rust function built and deployed is less that pleasant. This change incorporates documentation for a less manual workflow using the serverless framework for build/deployment quality life enhancements.

I split off deployment into its own doc section and created a subsection for manual building and setting up a local host and a subsection for using the serverless framework rust plugin which leverages the lambdaci project's provided docker images which faithfully reproduce the required build environment and wires some components together for a more seemless DX for those familiar with serverless framework workflows.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[davidbarsky]

Apologies for the delay in reviewing this PR—Stefano was on a well-deserved vacation and I had priorities at work that I couldn't ignore. This is great, but the copy-editor in me would like some wording changed.

[softprops]

pushed copy changes