Restructure Providers section to add an Opensource Platforms section

[rupakg]

Description

The current Providers section mixes commercial Serverless platforms with the opensource platforms like OpenWhisk, Kubeless, OpenFaaS etc.

Filename: /guide/source/providers/README.md

Page/Section: Providers -> Opensource Platforms

Contributing Author: Rupak Ganguly (Serverless Inc.)

Details:

• Add Opensource Platform to the TOC, and move Kubeless under it
• Add a subheading Opensource Platform
• Add the "A Comparative View" section under Opensource Platform with Kubeless, OpenFaaS in it.
• Move Kubeless content under this section
• Add OpenFaaS placeholder under it