[Right Rail] Component with GitHub buttons

[zstix]

Description

The docs site will have a "right rail" component that will show a few page-specific items.

The component will look similar to the right rail that appears on the developer site, so we will want to consider abstracting it to the theme. If we decide to do this, we will want the ability to toggle the "Page last modified" message and some other customization options.

NOTE: This tickets is just for making the component in the theme with the Create an issue and Edit this page buttons.
Adding the in-page navigation is tracked in another ticket: #192

NOTE: Mobile responsiveness is tracked in another ticket: #193

Acceptance Criteria

• Generalized right-rail component added to the Gatsby theme
• Right rail component re-added to the dev site
• Right rail component added to docs site pages
• GitHub buttons link to the correct locations
• Ability to set "last modified" as a prop
• Ability to set contributing guide (default to CONTRIBUTING.md)

Screenshot(s)

Right Rail on Developer Site:

Wireframes for Docs Site:

[zstix]

Move this to the theme repository.

[jerelmiller]

Created an issue in the theme repo for this as well: newrelic/gatsby-theme-newrelic#144

[jerelmiller]

This issue can now focus on only the docs work needed to integrate with the component.