feat(docs): Improve theming docs

[ryanseddon]

Description

The theming docs are currently a little light on detail when it comes to the naming used to target the components you wish to write theme overrides for.

Detail

This adds a new component called <CIDTable /> that renders a webpack global called COMPONENT_IDS.

On doc site build it scans all the js files for component_ids and injects it as a webpack global that is then consumed by <CIDTable /> to render them out.

This will make it automagically update anytime any new ids and added and/or changed.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 94.792% when pulling 70be94c on ryan/theming_doc_improvements into 74f2409 on master.

[austingreendev]

This is going to be very helpful! I may be crazy, but is it possible to extract this logic/table creation into the page build process.

Would the following be possible?

1. Add the file parsing script output as a webpack global (just a structured array)
2. Create a React example in the MD file that uses the provided global to spit out one of our react-tables?

Otherwise I could see us forgetting to keep this section up-to-date with every addition/retraction.

[jzempel]

If not added to the build, as Austin suggests, then at a minimum it should be added to the TODO comment section in https://github.com/zendeskgarden/react-components/blob/master/packages/.template/README.md (although this is not the ideal spot to catch all new COMPONENT_ID additions).

[austingreendev]

LGTM pending @jzempel comment