Add overview docs for molecule inner workings and plugin architecture

[sio]

Proposal: Add overview docs for molecule inner workings and plugin architecture

Author: Vitaly Potyarkin <@sio>

Date: 2020-02-07

• Status: New
• Proposal type: documentation
• Estimated time to implement: 1 day

Motivation

Molecule v3 moved a lot of features into separate projects (plugins) that are currently in need of new maintainers/developers. It is difficult for newcomers to get a clear picture of how Molecule works internally, even some hints at entry points from where to start code discovery would be helpful.

Problems

What problems exist that this proposal will solve?

• Lack of maintainers for plugins that were moved out of core Molecule project during v3 development
• Unusable state of some of the plugins after separating from the core codebase

Solution proposal

I suggest adding several new pages to Molecule documentation:

• High level overview of how Molecule works, maybe mentioning the key objects that hold state and initiate actions. Example: Pelican internals
• Introductory docs for plugin writers, maybe with a list of exemplary projects that are currently in a working state. Example: Pelican plugins

I've added two examples from Pelican project because they show that these docs do not have to be long and deep. Those two pages have helped me to quickly get started with my own plugin and to trace its behavior through all of the main project's source code.